timeline unit the atom · 2014-09-11 · his results demonstrated that elements combine in specific...

U N I T The Atom

Unit 4300

1911Ernest Rutherford, a physicist from

New Zealand, discovers the positively charged nucleus of the atom.

1932The neutron, one of the particles in the

nucleus of an atom, is discovered byBritish physicist James Chadwick.

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1964Scientists propose the idea that smaller particlesmake up protons and neutrons. The particles arenamed quarks after a word used by James Joyce

in his book Finnegans Wake.

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Around

400 B.C.The Greek philosopher Democritusproposes that small particles called

atoms make up all matter.

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1945The United Nations is

formed. Its purpose is tomaintain world peace anddevelop friendly relations

between countries.

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housands of yearsago, people began

asking the question,“What is matter madeof?” This unit followsthe discoveries andideas that have led toour current theoriesabout what makes upmatter. You will learnabout the atom—thebuilding block of allmatter—and its struc-ture. You will alsolearn how the periodictable is used to clas-sify and organize el-ements according topatterns in atomicstructure and otherproperties. This time-line illustrates some ofthe events that havebrought us to our cur-rent understanding ofatoms and of the peri-odic table in whichthey are organized.

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T I M E L I N E

Copyright © by Holt, Rinehart and Winston. All rights reserved.

The Atom 301

1848James Marshall finds gold

while building Sutter’sMill, starting the California

gold rush.

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1803British scientist and school

teacher John Dalton reintroduces the concept of

atoms with evidence tosupport his ideas.

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1869Russian chemist

Dmitri Mendeleevdevelops a periodictable that organizesthe elements known

at the time.

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!1898

British scientists Sir William Ramsay and

Morris W. Travers discoverthree elements—krypton,

neon, and xenon—in threemonths. The periodic tabledeveloped by Mendeleev

helps guide their research.

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1996Another element is added to the periodic tableafter a team of German scientists synthesize an

atom containing 112 protons in its nucleus.

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Germans celebrate when the BerlinWall ceases to function as a barrierbetween East and West Germany.

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1897British scientist J. J.

Thomson identifies elec-trons as particles that are

present in every atom.

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1981Scientists in Switzerland develop a

scanning tunneling microscope,which is used to see atoms for the

first time.

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Introduction to Atoms12C

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Would You Believe . . . ?Tiny atoms have something in common withhuge dinosaurs. In both cases, scientists havehad to try to understand something theycould not observe firsthand!

No one has ever seen a living dinosaur.So how did the special-effects crew for themovie Jurassic Park know what theTyrannosaurus rex model, shown above,should look like? Scientists have determinedthe appearance of T. rex by studying fos-silized skeletons. Based on fossil evidence,scientists theorize that these now-extinctcreatures had big hind legs, small front legs,a long whip-like tail, and an enormousmouth full of dagger-shaped teeth.

However, theories of how T. rex walkedhave been harder to develop because thereis no way to see a dinosaur in motion. Formany years, most scientists thought that T. rex (and all dinosaurs) plodded slowly like big, lazy lizards.

However, after studyingwell-preserved dinosaurtracks, like those shownbelow, and noticingskeletal similarities betweencertain dinosaur fossils and living creaturessuch as the ostrich, many scientists now the-orize that T. rex and its dinosaur cousinscould turn on the speed. Some scientistsestimate that T. rex had bursts of speed of32 km/h (20 mi/h)!

Theories about T. rex and other dinosaurshave changed gradually over many yearsbased on indirect evidence, such as dinosaurtracks. Likewise, our theory of the atom haschanged and grown over thousands of yearsas scientists have uncovered more evidenceabout the atom, even though they wereunable to see an atom directly. In this chap-ter, you’ll learn about the development ofthe atomic theory and our current under-standing of atomic structure.

302Copyright © by Holt, Rinehart and Winston. All rights reserved.

303

5. Continue rolling the marble from differentdirections to determine the shape and loca-tion of the object.

6. Write down all your observations in yourScienceLog.

Analysis7. Form a conclusion about the object’s shape,

size, and location. Record your conclusion inyour ScienceLog.

Where Is It?Theories about the internal structure of atoms weredeveloped by aiming moving particles at atoms.In this activity you will develop an idea about thelocation and size of a hidden object by rolling mar-bles at the object.

Procedure1. Place a rectangular piece of cardboard on four

books or blocks so that each corner of thecardboard rests on a book or block.

2. Ask your teacher to place the unknown objectunder the cardboard. Be sure that you do not see it.

3. Place a large piece of paper on top of the cardboard.

4. Gently roll a marble under the cardboard, andrecord on the paper the position where themarble enters and exits and the direction ittravels.

In your ScienceLog, try to answer thefollowing questions based on what youalready know:

1. What are some ways that scientistshave described the atom?

2. What are the parts of the atom, andhow are they arranged?

3. How are atoms of all elements alike?

Introduction to AtomsCopyright © by Holt, Rinehart and Winston. All rights reserved.

Chapter 12304

N E W T E R M Satom modeltheory nucleuselectrons electron clouds

O B J E CT I V E S! Describe some of the experi-

ments that led to the currentatomic theory.

! Compare the different modelsof the atom.

! Explain how the atomic theoryhas changed as scientists havediscovered new informationabout the atom.

Section1 Development of the

Atomic TheoryThe photo at right showsuranium atoms magnified 3.5 million times by a scan-ning tunneling microscope.An atom is the smallestparticle into which an ele-ment can be divided andstill be the same substance.Atoms make up elements;elements combine to formcompounds. Because allmatter is made of elementsor compounds, atoms areoften called the buildingblocks of matter.

Before the scanning tunneling microscope was invented,in 1981, no one had ever seen an atom. But the existence ofatoms is not a new idea. As you will find out, our under-standing of atoms has been developing for more than 2,000years. How is this possible? The answer has to do with theo-ries. A theory is a unifying explanation for a broad range ofhypotheses and observations that have been supported by test-ing. In this section, you will take a short trip through historyto see for yourself how our understanding of atoms developedeven before we could observe them directly. Your first stop—ancient Greece.

Democritus Proposes the AtomLook at the silver coin shown in Figure 1. Imagine that youcut the coin in half, then cut those halves in half, and so on.Could you keep cutting the pieces in half forever, or wouldyou eventually end up with a particle that you could not cut?

Around 440 B.C., a Greek philosopher named Democritus(di MAHK ruh tuhs) proposed that in such a situation, youwould end up with an “uncuttable” particle. He called thisparticle an atom (from the Greek word atomos, meaning “indi-visible”). Democritus proposed that all atoms are small, hardparticles made of a single material formed into different shapesand sizes. He also claimed that atoms are always moving andthat they form different materials by joining together.

Figure 1 Democritus thought thesmallest particle in an object likethis silver coin was an atom. Thiscoin was in use duringDemocritus’s time.


Aristotle (ER is TAHT uhl), a Greek philosopher who livedfrom 384 to 322 B.C., disagreed with Democritus’s ideas. Hebelieved that you could keep cutting an object in half overand over and never end up with an indivisible particle.Although Aristotle’s ideas were eventually proved incorrect, hehad such a strong influence on popular belief that Democritus’sideas were largely ignored for centuries.

Dalton Creates an Atomic Theory Based on ExperimentsBy the late 1700s, scientists had learned that elements combine in specific proportions to form compounds. Theseproportions are based on the mass of the elements in thecompounds. For example, hydrogen and oxygen always com-bine in the same proportion to form water. John Dalton, aBritish chemist and school teacher, wanted to know why. Heperformed experiments with different substances. His resultsdemonstrated that elements combine in specific proportionsbecause they are made of individual atoms. After many exper-iments and observations, Dalton, shown in Figure 2, pub-lished his own atomic theory in 1803. His theory stated thefollowing:

It took many years for scientists to accept Dalton’s atomictheory, but toward the end of the nineteenth century scien-tists agreed that his theory explained many of their observa-tions. However, as new information was discovered that could not be explained by Dalton’s ideas, the atomic theorychanged. The theory was revised to more correctly explain theatom. As you read on, you will learn how Dalton’s theory haschanged, step by step, into the current atomic theory.

Introduction to Atoms 305

All substances are made of atoms.Atoms are small particles that cannotbe created, divided, or destroyed.

Atoms of the same element are exactly alike, and atoms of different elements are different.

Atoms join with other atoms to make new substances.

Figure 2 John Dalton devel-oped his atomic theory fromobservations gathered frommany experiments.

In 342 or 343 B.C., KingPhillip II of Macedonappointed Aristotle to be atutor for his son, Alexander.Alexander later conqueredGreece and the PersianEmpire (in what is now Iran)and became known asAlexander the Great.


Thomson Finds Electrons in the AtomIn 1897, a British scientist named J. J. Thomson made a dis-covery that identified an error in Dalton’s theory. Using rela-tively simple equipment (compared with modern scientificequipment), Thomson discovered that there are small particlesinside the atom. Therefore, atoms can be divided into evensmaller parts. Atoms are not indivisible, as proposed by Dalton.

Thomson experimented with a cathode-ray tube, as shownin Figure 3. He discovered that the direction of the beam wasaffected by electrically charged plates. Notice in the illustra-tion that the plate marked with a positive sign, which repre-sents a positive charge, attracts the beam. Because the beamwas pulled toward a positive charge, Thomson concluded thatthe beam was made of particles with a negative electric charge.

Just What Is Electric Charge?Have you ever rubbed a balloon on your hair? The propertiesof your hair and the balloon seem to change, making themattract one another. To describe these observations, scientists

say that the balloon and your hair become “charged.” There aretwo types of charges, positive and negative. Objects with oppo-

site charges attract each other, while objects with the samecharge push each other away. When Thomson observed that thebeam was pulled toward a positively charged plate, he concludedthat the particles in the beam must be negatively charged.

Figure 3 Thomson’s Cathode-Ray Tube Experiment

Chapter 11306

When the plates werecharged, the beam pro-duced a glowing spot hereafter being pulled towardthe positively charged plate.

Metal plates could becharged to change thepath of the beam.

When the plates were not charged, thebeam produced a glowing spot here.Almost all gas was removed

from the glass tube.

An invisible beam wasproduced when the tube was connected to a source of electricalenergy.

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Thomson repeated his experiment several times and foundthat the particle beam behaved in exactly the same way eachtime. He called the particles in the beam corpuscles (KOR PUHS uhls). His results led him to conclude that cor-puscles are present in every type of atom and that all cor-puscles are identical. The negatively charged particles foundin all atoms are now called electrons.

Like Plums in a Pudding Thomson knew that electronswere a part of atoms and that Dalton’s belief that atoms couldnot be divided was therefore incorrect. Thomson revised theatomic theory to account for the presence of electrons, but hestill did not know how electrons are arranged inside atoms.In addition, chemists knew that atoms have no overall charge,so Thomson realized that positive charges must be present tobalance the negative charges of the electrons. But just asThomson didn’t know the location of the electrons, he didn’tknow the location of the positive charges. He proposed a modelto describe a possible structure of the atom. A model is arepresentation of an object or system. A model isdifferent from a theory in that a model presentsa picture of what the theory explains.

In Thomson’s model, illustrated inFigure 4, the atom is a positively chargedblob of material with electrons scatteredthroughout. This model came to be knownas the plum-pudding model, named for anEnglish dessert that was popular at thetime. The electrons could be compared tothe plums that were found throughout thepudding. Today you might call Thomson’smodel the chocolate-chip-ice-cream model;electrons in the atom could be comparedto the chocolate chips found throughoutthe ice cream!

Introduction to Atoms 307Introduction to Atoms

1. What discovery demonstrated that atoms are not thesmallest particles?

2. What did Dalton do in developing his theory thatDemocritus did not do?

3. Analyzing Methods Why was it important for Thomsonto repeat his experiment?

The word electron comesfrom a Greek word meaning“amber.” The ancientGreeks knew that a piece ofamber (the solidified sapfrom ancient trees) couldattract small bits of strawand paper after beingrubbed with cloth.

REVIEW

Thomson proposedthat the atom ismostly positivelycharged material.

In Thomson’smodel, electronsare small, nega-tively chargedparticles locatedthroughout the positive material.

Figure 4 Thomson’s plum-pudding model ofthe atom is shown above. A modern version ofThomson’s model might be chocolate-chip ice cream.


Figure 5 Rutherford’s Gold Foil Experiment

Chapter 12308

"It was quite the most incredible event that has ever happened to me in my life. It was almost as if you fired a fifteen-inch shell into a piece of tissue paper and it came back and hit you."

An element such as radium producedthe particles.

Very few particles seemedto bounce back.

Some particleswere slightlydeflected froma straight path.

Most of theparticles passedstraight throughthe gold foil.

Lead stopped all of the positiveparticles except for a small streamaimed at a gold foil target.

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Rutherford Opens an Atomic “Shooting Gallery”In 1909, a former student of Thomson’s named ErnestRutherford decided to test Thomson’s theory. He designed anexperiment to investigate the structure of the atom. He aimeda beam of small, positively charged particles at a thin sheetof gold foil. These particles were larger than protons, evensmaller positive particles identified in 1902. Even though thegold foil was thinner than the foil used to wrap a stick ofchewing gum, it was still about 10,000 atoms thick! Figure 5shows a diagram of Rutherford’s experiment. To find out wherethe particles went after being “shot” at the gold foil, Rutherfordsurrounded the foil with a screen coated with zinc sulfide, asubstance that glowed when struck by the particles.

Rutherford thought that if atoms were soft “blobs” of ma-terial, as suggested by Thomson, then the particles would passthrough the gold and continue in a straight line. Most of theparticles did just that. But to Rutherford’s great surprise, someof the particles were deflected (turned to one side) a little,some were deflected a great deal, and occasionally a particleseemed to bounce back. When describing his amazement,Rutherford reportedly said,

Find out aboutMelissa

Franklin, amodern atomexplorer, onpage 323.


Rutherford Presents a New Atomic Model It was obviousto Rutherford that the plum-pudding model of the atom didnot explain his results. In 1911, he revised the atomic theory.Rutherford concluded that because almost all of the particleshad passed through the gold foil, atoms are mostly emptyspace. He proposed that the lightweight, negative electronsmove in the empty space.

To explain the deflection of theother particles, Rutherford proposedthat in the center of the atom is a tiny,extremely dense, positively chargedregion called the nucleus (NOO kleeuhs). The Rutherford model is illus-trated in Figure 6. From the results ofhis experiment, Rutherford reasonedthat positively charged particles thatpassed close by the nucleus were pushedaway from their straight-line path bythe positive charges in the nucleus.(Remember, opposite charges attract,and like charges repel.) Occasionally, a particle would head straight for a nucleus and be pushed almost straightback in the direction from which it came.

From the results of Rutherford’s experiment, he calculatedthat the diameter of the nucleus was 100,000 times smallerthan the diameter of the gold atom. To imagine how smallthis is, look at Figure 7.

Figure 6 The results ofRutherford’s experimentled to a new model ofthe atom.

Because a few particles weredeflected by the foil, Rutherfordproposed that the atom has asmall, dense, positively chargednucleus. Most of the atom’smass is concentrated here.

Rutherford proposed thatbecause most particlespassed straight through thegold foil, the atom is mostlyempty space through whichelectrons travel.

Rutherford suspected thatelectrons travel around thenucleus like planets aroundthe sun, but he could notexplain the exact arrange-ment of the electrons.


Figure 7 The diameter ofthis pinhead is 100,000times smaller than thediameter of the stadium.Likewise, the diameter ofa nucleus is 100,000

times smaller than thediameter of an atom.

Self-CheckWhy did Thomsonbelieve the atom con-tains positive charges?(See page 596 to checkyour answer.)


Bohr States That Electrons Can Jump Between LevelsThe next step in understanding the atom came just 2 yearslater, from a Danish scientist who worked with Rutherford. In1913, Niels Bohr suggested that electrons travel around thenucleus in definite paths. These paths are located in levels atcertain distances from the nucleus, as illustrated in Figure 8.Bohr proposed that no paths are located between the levels,but electrons can jump from a path in one level to a path inanother level. Think of the levels as rungs on a ladder. Youcan stand on the rungs of a ladder but not between the rungs.

Bohr’s model was a valuable tool in predicting some atomicbehavior. But the model was too simple to explain all of thebehavior of atoms, so scientists continued to study the atomand improve the atomic theory.

The Modern Theory: Electron CloudsSurround the NucleusMany twentieth-century scientists have contributed to our cur-

rent understanding of the atom. An Austrian physicistnamed Erwin Schrödinger (1887–1961) and a German

physicist named Werner Heisenberg (1901–1976)made particularly important contributions. Theirwork further explained the nature of electrons inthe atom. For example, electrons do not travel indefinite paths as Bohr suggested. In fact, the exactpath of a moving electron cannot be predicted.According to the current theory, there are regions

inside the atom where electrons are likely to befound—these regions are called electron clouds.

Electron clouds are related to the paths described inBohr’s model. The electron-cloud model of the atom

is illustrated in Figure 9.

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Electron paths

Figure 8 Bohr proposed thatelectrons move in paths at certain distances around thenucleus.

1. In what part of an atom is most of its mass located?

2. What are two differences between the atomic theorydescribed by Thomson and that described by Rutherford?

3. Comparing Concepts Identify the difference in howBohr’s theory and the modern theory describe the loca-tion of electrons.

Chapter 12

Nucleus

Electron clouds

Nucleus

REVIEW

Figure 9 In the currentmodel of the atom, regions of the atom called electronclouds are the most likelyplaces to find electrons.



N E W T E R M Sprotonsatomic mass unit (amu)neutronsatomic numberisotopesmass numberatomic mass

O B J E CT I V E S! Compare the charge, location,

and relative mass of protons,neutrons, and electrons.

! Calculate the number of parti-cles in an atom using the atomicnumber, mass number, andoverall charge.

! Calculate the atomic mass ofelements.

Section2 The Atom

In the last section, you learned how the atomic theory devel-oped through centuries of observation and experimentation.Now it’s time to learn about the atom itself. In this section,you’ll learn about the particles inside the atom, and you’lllearn about the forces that act on those particles. But firstyou’ll find out just how small an atom really is.

How Small Is an Atom?The photograph below shows the pattern that forms when abeam of electrons is directed at a sample of aluminum. By ana-lyzing this pattern, scientists can determine the size of an atom.Analysis of similar patterns for many elements has shown thataluminum atoms, whichare average-sized atoms,have a diameter of about0.00000003 cm. That’s threehundred-millionths of acentimeter. That is so smallthat it would take a stack of 50,000 aluminum atomsto equal the thickness of a sheet of aluminum foilfrom your kitchen!

As another example, consider an ordinary penny.Believe it or not, a pennycontains about 2 ! 1022 atoms, which can be written as 20,000,000,000,000,000,000,000 atoms, of copper and zinc. That’s twenty thousand billion billion atoms—4,000,000,000,000 times more atoms than there are people on Earth! So if there are that many atoms in a penny, eachatom must be very small. You can get a better idea of just howsmall an atom is in Figure 10.

Figure 10 If you could enlarge a penny untilit was as wide as the continental United

States, each of its atoms would beonly about 3 cm in diameter—aboutthe size of this table-tennis ball.


What’s Inside an Atom?As tiny as an atom is, it consists of even smaller particles—protons, neutrons, and electrons—as shown in the model inFigure 11. (The particles represented in the figures are not shownin their correct proportions because the electrons would betoo small to see.) Protons and neutrons make up the nucleus,which is the center of the atom. Electrons are found outsidethe nucleus.

The Nucleus Protons are the positively charged particles ofthe nucleus. It was these particles that repelled Rutherford’s“bullets.” All protons are identical, and each proton has apositive charge. The mass of a proton is approximately1.7 ! 10"24g, which can also be written as0.0000000000000000000000017g. Because the masses of particles in atoms are so small, scientists developed a unit of measurement for them. The SI unit used to measure themasses of particles in atoms is the atomic mass unit (amu).Scientists assigned each proton a mass of 1 amu.

Neutrons are the particles of the nucleus that have nocharge. All neutrons are identical. Neutrons are slightly moremassive than protons, but the difference in mass is so smallthat neutrons are also given a mass of 1 amu.

Protons and neutrons are the most massive particles inan atom, yet the nucleus they form has a very small volume.In other words, the nucleus is very dense. In fact, if it werepossible for a nucleus to have a volume of 1 cm3—the vol-ume of an average grape—that nucleus would have a massgreater than 9 million metric tons!

Chapter 12312

Particle Profile

Name: proton

Charge: positive

Mass: 1 amu

Location: nucleus

Particle Profile

Name: neutron

Charge: none

Mass: 1 amu

Location: nucleus

Protons are positively charged particlesin the nucleus of an atom.

Neutrons are particles inthe nucleus of an atomthat have no charge.

Figure 11 An atom consists of three different types of particles—protons, neutrons, and electrons.

Electrons are negativelycharged particles found inelectron clouds outside thenucleus. The size of theelectron clouds determinesthe size of the atom.

The nucleus is the small,dense, positively chargedcenter of the atom. Itcontains most of theatom’s mass.

The diameter of thenucleus is 1/100,000 thediameter of the atom.


Outside of the Nucleus Electrons are the negatively chargedparticles in atoms. The current atomic theory states that elec-trons are found moving around the nucleus within electronclouds. The charges of protons and electrons are opposite butequal in size. Therefore, whenever there are equal numbers ofprotons and electrons, their charges cancel out. An atom hasno overall charge and is described as being neutral. If the num-ber of electrons is different from the number of protons, theatom becomes a charged particle called an ion (IE ahn). Ionsare positively charged if the protons outnumber the electrons,and they are negatively charged if the electrons outnumberthe protons.

Electrons are very small in mass compared with protonsand neutrons. It takes more than 1,800 electrons to equal themass of 1 proton. In fact, the mass of an electron is so smallthat it is usually considered to be zero.

How Do Atoms of Different Elements Differ?There are 112 different elements, each of which is madeof different atoms. What makes atoms different fromeach other? To find out, imagine that it’s possible to“build” an atom by putting together protons, neutrons,and electrons.

It’s easiest to start with the simplest atom. Protonsand electrons are found in all atoms, and the simplestatom consists of just one of each. It’s so simple it doesn’t even have a neutron. Put just oneproton in the center of the atom for thenucleus. Then put one electron in the electroncloud, as shown in the model in Figure 12.Because positive and negative charges in thisatom cancel each other out, your atom isneutral. Congratulations! You have just madethe simplest atom—a hydrogen atom.

1. What particles form the nucleus?

2.Explain why atoms are neutral.

3. Summarizing Data Why do scientists say that most ofthe mass of an atom is located in the nucleus?

REVIEW

Particle Profile

Name: electron

Charge: negative

Mass: almost zero

Location: electron clouds

Electron

Figure 12 The simplestatom has one protonand one electron.


Help wanted! Elements-4-Uneeds qualified nucleus

builders. Report to page 570 ofthe LabBook.

Proton


Now for Some Neutrons Now build an atom containingtwo protons. This time you will find that you must have someneutrons around to hold the protons together. Both of theprotons are positively charged, so they repel one another. Youcannot cram them together to form a nucleus unless you putsome neutrons there to counteract the repulsion. For this atom,two neutrons will do. Your new atom will have two protonsand two neutrons making up the nucleus and two electronszipping around outside the nucleus, as shown in the modelin Figure 13. This is an atom of the element helium.

You could continue combining particles, building all of the112 known elements. You could build a carbon atom using 6protons, 6 neutrons, and 6 electrons; you could build an oxy-gen atom using 8 protons, 9 neutrons, and 8 electrons; or youcould build an iron atom using 26 protons, 30 neutrons, and26 electrons. You could even build a gold atom with 79 pro-tons, 118 neutrons, and 79 electrons! As you can see, an atomdoes not have to have equal numbers of protons and neutrons.

The Number of Protons Determines the Element Howcan you tell which elements these atoms represent? The keyis the number of protons. The number of protons in the nucleusof an atom is the atomic number of that atom. Each elementis composed of atoms that all have the same atomic number.Every hydrogen atom has only one proton in its nucleus, sohydrogen has an atomic number of 1. Every carbon atom hassix protons in its nucleus, so carbon has an atomic numberof 6.

Are All Atoms of an Element the Same?Imagine you’re back in the atom-building work-shop. This time you’ll make an atom that hasone proton, one electron, and one neutron, asshown in Figure 14. This new atom has oneproton—what does that tell you? Its atomicnumber is 1, so it is hydrogen. This atom isneutral because there are equal numbers of pro-tons and electrons. However, this hydrogenatom’s nucleus has two particles; therefore, thisatom has a greater mass than the first hydro-gen atom you made. What you have is anotherisotope (IE suh TOHP) of hydrogen.

Chapter 12314

ProtonElectron

Neutron

astronomyC O N N E C T I O N

Hydrogen is the most abun-dant element in the universe.It is the fuel for the sun andother stars. It is currentlybelieved that there are roughly2,000 times more hydrogenatoms than oxygen atoms and10,000 times more hydrogenatoms than carbon atoms.

Figure 13 A helium nucleusmust have neutrons in it to keepthe protons from moving apart.

NeutronElectronProton

Figure 14 The atom in this modeland the one in Figure 12 are isotopesbecause each has one proton but adifferent number of neutrons.


Isotopes are atoms that have thesame number of protons but havedifferent numbers of neutrons. Eachelement has a limited number ofisotopes that occur naturally. Atomsthat are isotopes of each other arealways the same element becausethe number of protons in each atomis the same.

Some isotopes of an elementhave unique properties because theyare unstable. An unstable atom isan atom whose nucleus can changeits composition. This type of isotopeis radioactive. However, isotopes ofan element share most of the samechemical and physical properties.For example, the most common oxygen isotope has 8 neu-trons in the nucleus, but other isotopes have 9 or 10 neutrons.All three isotopes are colorless, odorless gases at room tem-perature. Each isotope has the chemical property of combin-ing with a substance as it burns and even behaves the samein chemical changes in your body.

How Can You Tell One Isotope from Another? You canidentify each isotope of an element by its mass number. Themass number is the sum of the protons and neutrons in an atom.Electrons are not included in an atom’s mass number becausetheir mass is so small that they have very little effect on theatom’s total mass. Look at the boron isotope models shownin Figure 15 to see how to calculate an atom’s mass number.


Figure 15 Each of theseboron isotopes has five pro-tons. But because each hasa different number of neu-trons, each has a differentmass number.

Oxygen reacts, or undergoesa chemical change, with the

hot filament in a light bulb, causingthe bulb to burn out quickly. Theelement argon does not react with thefilament, so a light bulb filled with argon doesnot burn out as quickly as one that containsoxygen. Three isotopes of argon occur in nature. Do youthink all three isotopes have the same effect on the filament when used in light bulbs? Explain your reasoning.

Protons: 5Neutrons: 5Electrons: 5Mass number =protons + neutrons = 10

Protons: 5Neutrons: 6Electrons: 5Mass number =protons + neutrons = 11


To identify a specific isotope of an element, write the nameof the element followed by a hyphen and the mass numberof the isotope. A hydrogen atom with one proton and no neu-trons has a mass number of 1. Its name is hydrogen-1.Hydrogen-2 has one proton and one neutron in the nucleus.The carbon isotope with a mass number of 12 is called car-bon-12. If you know that the atomic number for carbon is 6,you can calculate the number of neutrons in carbon-12 bysubtracting the atomic number from the mass number. Forcarbon-12, the number of neutrons is 12 " 6, or 6.

12 Mass number"6 Number of protons (atomic number)

6 Number of neutrons

How Do You Calculate the Mass of an Element?Most elements found in nature contain a mixture of differentisotopes. For example, all copper is composed of copper-63atoms and copper-65 atoms. The term atomic mass describesthe mass of a mixture of isotopes. Atomic mass is the weightedaverage of the masses of all the naturally occurring isotopesof an element. A weighted average accounts for the percent-ages of each isotope that are present. Copper, including thecopper in the Statue of Liberty (shown in Figure 16), is 69 percent copper-63 and 31 percent copper-65. The atomicmass of copper is 63.6 amu. Because the atomic mass is closerto 63 than to 65, you cantell the percentage of cop-per-63 is greater than thepercentage of copper-65.

Most elements have twoor more stable (nonradio-active) isotopes found innature. Tin has 10 stable iso-topes, which is more thanany other element. You cantry your hand at calculatingatomic mass by doing theMathBreak at left.

Chapter 12316

Figure 16 The copper usedto make the Statue of Libertyincludes both copper-63 andcopper-65. Copper’s atomicmass is 63.6 amu.

Atomic MassTo calculate the atomic massof an element, multiply themass number of each isotopeby its percentage abundancein decimal form. Then addthese amounts together tofind the atomic mass. Forexample, chlorine-35 makesup 76 percent (its percent-age abundance) of all thechlorine in nature, andchlorine-37 makes up theother 24 percent. The atomicmass of chlorine is calculatedas follows:

Now It’s Your TurnCalculate the atomic mass ofboron, which occurs naturallyas 20 percent boron-10 and80 percent boron-11.

MATH BREAK

(35 ! 0.76) # 26.6(37 ! 0.24) # $8.9

35.5 amu

Draw diagrams of hydrogen-2,helium-3, and carbon-14.Show the correct number andlocation of each type of parti-cle. For the electrons, simplywrite the total number ofelectrons in the electroncloud. Use colored pencils ormarkers to represent the pro-tons, neutrons, and electrons.


What Forces Are at Work in Atoms?You have seen how atoms are composed of protons, neutrons,and electrons. But what are the forces (the pushes or pullsbetween two objects) acting between these particles? Four basicforces are at work everywhere, including within the atom—gravity, the electromagnetic force, the strong force, and theweak force. These forces are discussed below.


1. List the charge, location, and mass of a proton, a neutron,and an electron.

2. Determine the number of protons, neutrons, and elec-trons in an atom of aluminum-27.

3.Doing Calculations The metal thallium occurs natu-rally as 30 percent thallium-203 and 70 percent thal-lium-205. Calculate the atomic mass of thallium.

REVIEW

Weak Force The weak force is an important force in radioactive atoms. In certain unstable atoms, a neutron can change into a proton and an electron. The weak force plays a key role in this change.

Strong Force Protons push away from one another because of the electromagnetic force. A nucleus containing two or more protonswould fly apart if it were not for the strong force. At the close distances between protons in the nucleus, the strong force is greaterthan the electromagnetic force, so the nucleus stays together.

Electromagnetic Force As mentioned earlier, objects that have the same charge repel each other, while objects with opposite charge attract each other. This is due to the electromagnetic force. Protons and electrons are attracted to each other because they have opposite charges. The electromagnetic force holds the electrons around the nucleus.

Gravity Probably the most familiar of the four forces is gravity. Gravityacts between all objects all the time. The amount of gravity betweenobjects depends on their masses and the distance between them. Gravitypulls objects, such as the sun, Earth, cars, and books, toward one another.However, because the masses of particles in atoms are so small, the force of gravity within atoms is very small.

Particles with the samecharges repel each other.

Particles with oppositecharges attract each other.

!

!!

Forces in the Atom

!"

!

!

! !

!

!

!

!


Chapter Highlights

Chapter 12318

SECTION 1

Skills Check

Vocabularyatom (p. 304)theory (p. 304)electrons (p. 307)model (p. 307)nucleus (p. 309)electron clouds (p. 310)

Section Notes• Atoms are the smallest parti-

cles of an element that retainthe properties of the element.

• In ancient Greece,Democritus argued thatatoms were the smallest par-ticles in all matter.

• Dalton proposed an atomictheory that stated the follow-ing: Atoms are small particlesthat make up all matter;atoms cannot be created,divided, or destroyed; atomsof an element are exactlyalike; atoms of different ele-ments are different; andatoms join together to makenew substances.

• Thomson discovered elec-trons. His plum-puddingmodel described the atom asa lump of positively chargedmaterial with negative elec-trons scattered throughout.

• Rutherford discovered thatatoms contain a small, dense,positively charged centercalled the nucleus.

• Bohr suggested that electronsmove around the nucleus atonly certain distances.

• According to the currentatomic theory, electronclouds are where electronsare most likely to be in thespace around the nucleus.

Math ConceptsATOMIC MASS The atomic mass of an elementtakes into account the mass of each isotope andthe percentage of the element that exists as thatisotope. For example, magnesium occurs natu-rally as 79 percent magnesium-24, 10 percentmagnesium-25, and 11 percent magnesium-26.The atomic mass is calculated as follows:

(24 ! 0.79) " 19.0(25 ! 0.10) " 2.5(26 ! 0.11) " # 2.8

24.3 amu

Visual UnderstandingATOMIC MODELSThe atomic theory haschanged over the pastseveral hundred years. Tounderstand the differentmodels of the atom, lookover Figures 2, 4, 6, 8, and 9.

PARTS OF THE ATOM Atoms are composed ofprotons, neutrons, and electrons. To review theparticles and their placement in the atom,study Figure 11 on page 312.


319Introduction to Atoms

SECTION 2

GO TO: go.hrw.com GO TO: www.scilinks.org

Visit the National Science Teachers Association on-line Website for Internet resources related to this chapter. Just type inthe sciLINKS number for more information about the topic:

TOPIC: Development of the Atomic Theory sciLINKS NUMBER: HSTP255TOPIC: Modern Atomic Theory sciLINKS NUMBER: HSTP260TOPIC: Inside the Atom sciLINKS NUMBER: HSTP265TOPIC: Isotopes sciLINKS NUMBER: HSTP270

Visit the HRW Web site for a variety oflearning tools related to this chapter. Just type in the keyword:

KEYWORD: HSTATS

Vocabularyprotons (p. 312)atomic mass unit (amu) (p. 312)neutrons (p. 312)atomic number (p. 314)isotopes (p. 315)mass number (p. 315)atomic mass (p. 316)

Section Notes• A proton is a positively

charged particle with a massof 1 amu.

• A neutron is a particle withno charge that has a mass of1 amu.

• An electron is a negativelycharged particle with anextremely small mass.

• Protons and neutrons makeup the nucleus. Electrons arefound in electron clouds out-side the nucleus.

• The number of protons inthe nucleus of an atom is theatomic number. The atomicnumber identifies the atomsof a particular element.

• Isotopes of an atom have thesame number of protons buthave different numbers ofneutrons. Isotopes sharemost of the same chemicaland physical properties.

• The mass number of an atomis the sum of the atom’s neu-trons and protons.

• The atomic mass is an aver-age of the masses of all naturally occurring isotopesof an element.

• The four forces at work in anatom are gravity, the electro-magnetic force, the strongforce, and the weak force.

LabsMade to Order (p. 570)


Chapter ReviewUSING VOCABULARY

The statements below are false. For each state-ment, replace the underlined word to make atrue statement.

1. Electrons are found in the nucleus of anatom.

2. All atoms of the same element containthe same number of neutrons.

3. Protons have no electrical charge.

4. The atomic number of an element is thenumber of protons and neutrons in thenucleus.

5. The mass number is an average of themasses of all naturally occurring isotopesof an element.

UNDERSTANDING CONCEPTS

Multiple Choice

6. The discovery of which particle provedthat the atom is not indivisible?a. proton c. electronb. neutron d. nucleus

7. In his gold foil experiment, Rutherfordconcluded that the atom is mostly emptyspace with a small, massive, positivelycharged center becausea. most of the particles passed straight

through the foil.b. some particles were slightly deflected.c. a few particles bounced back.d. All of the above

8. How many protons does an atom with anatomic number of 23 and a mass numberof 51 have?a. 23 c. 51b. 28 d. 74

9. An atom has no overall charge if it con-tains equal numbers ofa. electrons and protons.b. neutrons and protons.c. neutrons and electrons.d. None of the above

10. Which statement about protons is true?a. Protons have a mass of 1/1840 amu.b. Protons have no charge.c. Protons are part of the nucleus of an

atom.d. Protons circle the nucleus of an atom.

11. Which statement about neutrons is true?a. Neutrons have a mass of 1 amu.b. Neutrons circle the nucleus of an atom.c. Neutrons are the only particles that

make up the nucleus.d. Neutrons have a negative charge.

12. Which of the following determines theidentity of an element?a. atomic number c. atomic massb. mass number d. overall charge

13. Isotopes exist because atoms of the sameelement can have different numbers ofa. protons. c. electrons.b. neutrons. d. None of the above

Short Answer

14. Why do scientific theories change?

15. What force holds electrons in atoms?

16. In two or three sentences, describe theplum-pudding model of the atom.

Chapter 12320Copyright © by Holt, Rinehart and Winston. All rights reserved.

Concept Mapping

17. Use the followingterms to create a concept map: atom,nucleus, protons,neutrons, electrons,isotopes, atomic num-ber, mass number.

CRITICAL THINKING AND PROBLEM SOLVING

18. Particle accelerators, like the one shownbelow, are devices that speed up chargedparticles in order to smash them together.Sometimes the result of the collision is anew nucleus. How can scientists deter-mine whether the nucleus formed is thatof a new element or that of a new isotopeof a known element?

19. John Dalton made a number of state-ments about atoms that are now knownto be incorrect. Why do you think hisatomic theory is still found in sciencetextbooks?

MATH IN SCIENCE

20. Calculate the atomic mass of gallium consisting of 60 percent gallium-69 and40 percent gallium-71.

21. Calculate the number of protons, neu-trons, and electrons in an atom of zirconium-90, which has an atomic num-ber of 40.

INTERPRETING GRAPHICS

22. Study the models below, and answer thequestions that follow:

a. Which models represent isotopes of thesame element?

b. What is the atomic number for (a)?c. What is the mass number for (b)?

23. Predict how the direction of the movingparticle in the figure below will change,and explain what causes the change tooccur.

Key

Proton

Neutron

Electron

a b

c

Take a minute to review your answersto the ScienceLog questions on page303. Have your answers changed? Ifnecessary, revise your answers based on what you have learned since youbegan this chapter.

321Introduction to AtomsCopyright © by Holt, Rinehart and Winston. All rights reserved.

P H Y S I C A L S C I E N C E • A S T R O N O M Y

Water on the Moon?

322

When the astronauts of the Apollo space missionexplored the surface of the moon in 1969, allthey found was rock powder. None of the manysamples of moon rocks they carried back toEarth contained any hint of water. Because theastronauts didn’t see water on the moon andscientists didn't detect any in the lab, scientistsbelieved there was no water on the moon.

Then in 1994, radio waves suggested anotherpossibility. On a 4-month lunar jaunt, anAmerican spacecraft called Clementine beamedradio waves toward various areas of the moon,including a few craters that never receive sun-light. Mostly, the radio waves were reflected bywhat appeared to be ground-up rock. However,in part of one huge, dark crater, the radiowaves were reflected as if by . . . ice.

Hunting for Hydrogen AtomsScientists were intrigued by Clementine’sevidence. Two years later, another spacecraft,Lunar Prospector, traveled to the moon. Insteadof trying to detect water with radio waves,Prospector scanned the moon’s surface with adevice called a neutron spectrometer (NS). Aneutron spectrometer counts the number ofslow neutrons bouncing off a surface. When aneutron hits something about the same massas itself, it slows down. As it turns out, the onlything close to the mass of a neutron is an atomof the lightest of all elements, hydrogen. Sowhen the NS located high concentrations ofslow-moving neutrons on the moon, it indi-cated to scientists that the neutrons werecrashing into hydrogen atoms.

As you know, water consists of two atoms ofhydrogen and one atom of oxygen. The presenceof hydrogen atoms on the moon is more evi-dence that water may exist there.

How Did It Get There?Some scientists speculate that the water mol-ecules came from comets (which are 90 per-cent water) that hit the moon more than 4 billion years ago. Water from comets mayhave landed in the frigid, shadowed craters ofthe moon, where it mixed with the soil andfroze. The Aitken Basin, at the south pole of themoon, where much of the ice was detected, ismore than 12 km deep in places. Sunlightnever touches most of the crater. And it is verycold—temperatures there may fall to !229"C.The conditions seem right to lock water intoplace for a very long time.

Think About Lunar Life! Do some research on conditions on themoon. What conditions would humans have toovercome before we could establish a colonythere?

" The Lunar Prospector spacecraft may havefound water on the moon.


323

M elissa Franklin is an experimental physicist. “I am trying tounderstand the forces that describe how everything in the

world moves—especially the smallest things,” she explains. “I wantto find the things that make up all matter in the universe and thentry to understand the forces between them.”

Other scientists rely on her to test some of the most importanthypotheses in physics. For instance, Franklin and her team recentlycontributed to the discovery of a particle called the top quark.(Quarks are the tiny particles that make up protons and neutrons.)

Physicists had theorized that the top quark might exist but hadno evidence. Franklin and more than 450 other scientists workedtogether to prove the existence of the top quark. Finding itrequired the use of a massive machine called a particle accelerator.Basically, a particle accelerator smashes particles together, and thenscientists look for the remains of the collision. The physicists had tobuild some very complicated machines to detect the top quark, butthe discovery was worth the effort. Franklin and the otherresearchers have earned the praise of scientists all over the world.

Getting Her Start“I didn’t always want to be a scientist, but what happens is thatwhen you get hooked, you really get hooked. The next thing youknow, you’re driving forklifts and using overhead cranes while at

In the course of a single day, you could find MelissaFranklin operating a hugedrill, giving a tour of her lab toa 10-year-old, putting togethera gigantic piece of electronicequipment, or even telling ajoke. Then you’d see her reallyget down to business—studying the smallest particlesof matter in the universe.

EXPERIMENTAL PHYSICIST

the same time working on really tiny, incrediblycomplicated electronics. What I do is a combinationof exciting things. It’s better than watching TV.”

It isn’t just the best students who grow up to bescientists. “You can understand the ideas withouthaving to be a math genius,” Franklin says. Anyonecan have good ideas, she says, absolutely anyone.

Don’t Be Shy!! Franklin also has some good advice for youngpeople interested in physics. “Go and bug peopleat the local university. Just call up a physics personand say, ‘Can I come visit you for a couple ofhours?’ Kids do that with me, and it’s really fun.”Why don't you give it a try? Prepare for the visit bymaking a list of questions you would like answered.

" This particle accelerator was used inthe discovery of the top quark.


Chapter 13324

CH

AP

TE

R

Suppose someone told you that the smallanimal shown above—a yellow-spotted rockhyrax—is genetically related to an elephant.Impossible, you say? But it’s true! Eventhough this animal looks more like a rabbitor a rodent, scientists have determinedthrough DNA studies that the closest rela-tives of the hyrax are aardvarks, sea cows,and elephants. Biologists have uncoveredsimilar genetic links between other seem-ingly different species.

Scientists have also discovered that manydifferent-looking elements, like those shownat right, actually have common properties.For almost 150 years, scientists have organ-ized elements by observing the similarities(both obvious and not so obvious) betweenthem. One scientist in particular—a Russiannamed Dmitri Mendeleev (MEN duh LAY uhf)—organized the known elements in such a waythat a repeating pattern emerged. Mendeleevactually used this pattern to predict the prop-erties of elements that had not even been

discovered! His method of organizationbecame known as the periodic table.

The modern periodic table is arrangedsomewhat differently than Mendeleev’s, butit is still a useful tool for organizing the knownelements and predicting theproperties of elements stillunknown. Read on to learnabout the development ofthis remarkable table andthe patterns it reveals.

13

Although solid iodine and liquid bromine havevery different appearances, they have similarchemical properties.

The Periodic Table

Would You Believe . . . ?


The Periodic Table 325

Placement PatternJust as with animals, scientists have found patternsamong the elements. You too can find patterns—right in your classroom! By gathering and analyzinginformation about your classmates, you can deter-mine the pattern behind a new seating chart yourteacher has created.

Procedure1. In your ScienceLog, draw a seating chart for the

classroom arrangement designated by yourteacher. Write the name of each of your class-mates in the correct place on the chart.

2. Write information about yourself, such as yourname, date of birth, hair color, and height, inthe space that represents you on the chart.

3. Starting with the people around you, ask ques-tions to gather the same type of informationabout them. Write information about each per-son in the corresponding spaces on the seat-ing chart.

Analysis4. In your ScienceLog, identify a pattern within the

information you gathered that could be usedto explain the order of people in the seatingchart. If you cannot find a pattern, collect moreinformation, and look again.

5. Test your pattern by gathering information froma person you did not talk to before.

6. If the new information does not support yourpattern, reanalyze your data, and collect moreinformation as needed to determine anotherpattern.

Going FurtherThe science of classifying organisms is called taxonomy. Find out more about the way the Swedishscientist Carolus Linnaeus classified organisms.

In your ScienceLog, try to answer thefollowing questions based on what youalready know:

1. How are elements organized in theperiodic table?

2. Why is the table of the elementscalled “periodic”?

3. What one property is shared by el-ements in a group?


Chapter 13326

N E W T E R M Speriodic periodperiodic law group

O B J E CT I V E S! Describe how elements are

arranged in the periodic table.! Compare metals, nonmetals, and

metalloids based on their prop-erties and on their location inthe periodic table.

! Describe the difference betweena period and a group.

Section1 Arranging the Elements

Imagine you go to a new grocery store in your neighborhoodto buy a box of cereal. You are surprised by what you findwhen you walk into the store. None of the aisles are labeled,and there is no pattern to the products on the shelves! In frus-tration, you think it might take you days to find your cereal.

Some scientists probably felt a similar frustration before1869. By that time, more than 60 different elements had beendiscovered and described. However, the elements were notorganized in any special way. But in 1869, the elements wereorganized into a table in much the same way products arearranged (usually!) by shelf and aisle in a grocery store.

Discovering a PatternIn the 1860s, a Russian chemist named Dmitri Mendeleev

began looking for patterns among the properties of theknown elements. He wrote the names and properties

of these elements on small pieces of paper. Heincluded information such as density, appearance,

atomic mass, melting point, and any infor-mation he had about the compounds

formed from the element. He thenarranged and rearranged the pieces ofpaper, as shown in Figure 1. Aftermuch thought and work, he deter-mined that there was a repeating pat-tern to the properties of the elementswhen the elements were arranged inorder of increasing atomic mass.

The Properties of Elements Are Periodic Mendeleev sawthat the properties of the elements were periodic, meaning theyhad a regular, repeating pattern. Many things that are familiarto you are periodic. For example, the days of the week are peri-odic because they repeat in the same order every 7 days.

When the elements were arranged in order of increasingatomic mass, similar chemical and physical properties wereobserved in every eighth element. Mendeleev’s arrangementof the elements came to be known as a periodic table becausethe properties of the elements change in a periodic way.

Figure 1 By playing “chemical solitaire” on long train rides,Mendeleev organized the elements according to their properties.

In your ScienceLog, make alist of five things that are peri-odic. Explain which repeatingproperty causes each one tobe periodic.


Predicting Properties of Missing Elements Look atMendeleev’s periodic table in Figure 2. Notice the questionmarks. Mendeleev recognized that there were elements mis-sing. Instead of questioning his arrangement, he boldly pre-dicted that elements yet to be discovered would fill the gaps.He also predicted the properties of the missing elements byusing the pattern of properties in the periodic table. Whenone of the missing elements, gallium, was discovered a fewyears later, its properties matched Mendeleev’s predictions verywell, and other scientists became interested in his work. Sincethat time, all of the missing elements on Mendeleev’s periodictable have been discovered. In the chart below, you can seeMendeleev’s predictions for another missing element—germa-nium—and the actual properties of that element.

Changing the ArrangementMendeleev noticed that a few elements in the table were notin the correct place according to their properties. He thoughtthat the calculated atomic masses were incorrect and that moreaccurate atomic masses would eventually be determined.However, new measurements of the atomic masses showedthat the masses were in fact correct.

The mystery was solved in 1914 by a British scientist namedHenry Moseley (MOHZ lee). From the results of his experi-ments, Moseley was able to determine the number of pro-tons—the atomic number—in an atom. When he rearrangedthe elements by atomic number, every element fell into itsproper place in an improved periodic table.

Since Moseley’s rearrangement of the elements, more el-ements have been discovered. The discovery of each new el-ement has supported the periodic law, considered to be thebasis of the periodic table. The periodic law states that thechemical and physical properties of elements are periodicfunctions of their atomic numbers. The modern version of theperiodic table is shown on the following pages.


Properties of Germanium

Mendeleev’s Actual predictions properties

Atomic mass 72 72.6

Density 5.5 g/cm3 5.3 g/cm3

Appearance dark gray metal gray metal

Melting point high melting point 937!C

Figure 2 Mendeleev used ques-tion marks to indicate some el-ements that he believed wouldlater be identified.

Moseley was 26 when hemade his discovery. Hiswork allowed him to predictthat only three elementswere yet to be foundbetween aluminum andgold. The following year, ashe fought for the British inWorld War I, he was killed inaction at Gallipoli, Turkey.The British government nolonger assigns scientists tocombat duty.


140.1

232.0

140.9

231.0

144.2

238.0

(144.9)

(237.0)

150.4

244.1

6.9

23.0

39.1

85.5

132.9

(223.0)

9.0

24.3

40.1

87.6

137.3

(226.0)

45.0

88.9

138.9

(227.0)

47.9

91.2

178.5

(261.1)

50.9

92.9

180.9

(262.1)

52.0

95.9

183.8

(263.1)

54.9

(97.9)

186.2

(262.1)

55.8

101.1

190.2

(265)

58.9

102.9

192.2

(266)

1.0

Praseodymium

Rutherfordium

Molybdenum

Lithium

Sodium

Potassium

Rubidium

Cesium

Francium

Cerium

Thorium Protactinium

Neodymium

Uranium

Promethium

Neptunium

Samarium

Plutonium

Beryllium

Magnesium

Calcium

Strontium

Barium

Radium

Scandium

Yttrium

Lanthanum

Actinium

Titanium

Zirconium

Hafnium

Vanadium

Niobium

Tantalum

Dubnium

Chromium

Tungsten

Seaborgium

Manganese

Technetium

Rhenium

Bohrium

Iron

Ruthenium

Osmium

Hassium

Cobalt

Rhodium

Iridium

Meitnerium

Hydrogen

Li

V

Na

K

Rb

Cs

Fr

Be

Mg

Ca

Sr

Ba

Ra

Sc

Y

La

Ac

Ti

Zr

H f

Rf

Nb

Ta

Db

Cr

Mo

W

Sg

Mn

Re

Bh

IrOs

Ce

Th

Pr

Pa

Nd

U

Pm

Np

Sm

Pu

Fe

Ru

Hs

Co

Rh

Mt

H

Tc

3

11

19

37

55

87

58

90

59

91

60

92

61

93

62

94

4

12

20

38

56

88

21

39

57

89

22

40

72

104

23

41

73

105

24

42

74

106

25

43

75

107

26

44

76 77

108

27

45

109

1

Group 3 Group 4 Group 5 Group 6 Group 7 Group 8 Group 9

Group 1 Group 2

Period 1

Period 2

Period 3

Period 4

Period 5

Period 6

Period 7

Lanthanides

Actinides

BackgroundMetals

Metalloids

Nonmetals

Chemical symbolSolid

Liquid

Gas

6

CCarbon

12.0

Periodic Tableof the ElementsEach square on the table includes anelement’s name, chemical symbol,atomic number, and atomic mass.

Atomic number

Chemical symbol

Element name

Atomic mass

Chapter 13328

The color of the chemicalsymbol indicates the physicalstate at room temperature.Carbon is a solid.

The background colorindicates the type ofelement. Carbon is anonmetal.

A column of el-ements is called agroup or family.

A row of elements iscalled a period.

These elements are placed below thetable to allow the table to be narrower.


152.0

(243.1)

157.3

(247.1)

158.9

(247.1)

162.5

(251.1)

164.9

(252.1)

167.3

(257.1)

168.9

(258.1)

173.0

(259.1)

175.0

(262.1)

58.7 63.5 65.4 69.7 72.6 74.9 79.0 79.9 83.8

27.0 28.1 31.0 32.1 35.5 39.9

10.8 12.0 14.0 16.0 19.0 20.2

4.0

106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.9 131.3

195.1

(271) (272)

197.0 200.6 204.4 207.2 209.0 (209.0) (210.0) (222.0)

(277)

Europium

Americium

Gadolinium

Curium

Terbium

Berkelium

Dysprosium

Californium

Holmium

Einsteinium

Erbium

Fermium

Thulium

Mendelevium

Ytterbium

Nobelium

Lutetium

Lawrencium

Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton

Aluminum Silicon Phosphorus Sulfur Chlorine Argon

Boron Carbon Nitrogen Oxygen Fluorine Neon

Helium

Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon

Ununnilium Unununium

Platinum Gold Mercury Thallium Lead Bismuth Polonium Astatine Radon

Ununbium

Eu

Am

Gd

Cm

Tb

Bk

Dy

Cf

Pd Ag Cd In Sn Sb Te I Xe

Pt

Uun Uuu

Au Hg Tl Pb Bi Po At Rn

Ho

Es

Er

Fm

Tm

Md

Yb

No

Lu

Lr

Ni Cu Zn Ga Ge As Se Br Kr

Al Si P S Cl Ar

B C N O F Ne

He

Uub

28 29 30 31 32 33 34 35 36

13 14 15 16 17 18

5 6 7 8 9 10

2

46 47 48 49 50 51 52 53 54

78 79 80 81 82 83 84 85 86

110 111

63

95

64

96

65

97

66

98

67

99

68

100

69

101

70

102

71

103

112

Group 13 Group 14 Group 15 Group 16 Group 17

Group 18

Group 10 Group 11 Group 12


A number in parentheses is the mass numberof the most stable isotope of that element.

The names and symbols of elements 110–112 aretemporary. They are based on the atomic number ofthe element. The official name and symbol will beapproved by an international committee of scientists.

This zigzag linereminds you wherethe metals, nonmetals,and metalloids are.


Finding Your Way Around the Periodic TableAt first glance, you might think studying the periodic table islike trying to explore a thick jungle without a guide—it wouldbe easy to get lost! However, the table itself contains a lot ofinformation that will help you along the way.

Classes of Elements Elements are classified as metals,nonmetals, and metalloids, according to their properties. Thenumber of electrons in the outer energy level of an atomalso helps determine which category an element belongs in.The zigzag line on the periodic table can help you recog-nize which elements are metals, which are nonmetals, andwhich are metalloids.

Most of the elements in the periodictable are metals. Metals are found to theleft of the zigzag line on the periodictable. Atoms of most metals have fewelectrons in their outer energy level, asshown at right.

Most metals are solid at room tem-perature. Mercury, however, is a liquid.Some additional information on propertiesshared by most metals is shown below.

Metals

A model of amagnesium atom

Most metals are malleable, meaningthat they can be flattened with a hammer without shattering.Aluminum is flattened intosheets to make cans and foil.

330

Most metals are ductile, whichmeans that they can be drawninto thin wires. All metals aregood conductors of electriccurrent. The wires in the electrical devices in yourhome are made fromthe metal copper.

Chapter 13

Metals tend to be shiny. Youcan see a reflection in a mirrorbecause light reflects off theshiny surface of a thin layer ofsilver behind the glass.

Most metals are goodconductors of thermalenergy. This iron griddleconducts thermal energyfrom a stovetop to cookyour favorite foods.



Nonmetals are found to the right of thezigzag line on the periodic table. Atoms ofmost nonmetals have an almost completeset of electrons in their outer level, asshown at right. (Atoms of one group ofnonmetals, the noble gases, have a com-plete set of electrons, with most havingeight electrons in their outer energy level.)

More than half of the nonmetals aregases at room temperature. The prop-erties of nonmetals are the opposite ofthe properties of metals, as shown below.

Nonmetals

Metalloids, also called semiconductors,are the elements that border the zigzagline on the periodic table. Atoms of met-alloids have about a half-complete set ofelectrons in their outer energy level, asshown at right.

Metalloids have some properties ofmetals and some properties of nonmetals,as shown below.

Metalloids

A model of asilicon atom

A model of achlorine atom

Nonmetals are poor conductors of thermal energy and electriccurrent. If the gap in a sparkplug is too wide, the non-metals nitrogen and oxygen in

the air will stop the spark, and acar’s engine will not run.

Nonmetals are not malleable or ductile. In fact, solid nonmetals, like carbon (shown here in the graphite of the pencil lead), are brittle and will break or shatter when hit with a hammer.

Tellurium is shiny, but it is also brittleand is easily smashed into a powder.

Conduction Connection1. Fill a clear plastic

cup with hot water.

2. Stand a piece ofcopper wire and agraphite lead from amechanical pencil in thewater.

3. After 1 minute, touch thetop of each object. Recordyour observations.

4. Which material conductedthermal energy the best?Why?

Boron is almost as hardas diamond, but it isalso very brittle. At hightemperatures, boron isa good conductor ofelectric current.

Sulfur, like most nonmetals,is not shiny.


Each Element Is Identified by a Chemical Symbol Eachsquare on the periodic table contains information about anelement, including its atomic number, atomic mass, name, andchemical symbol. An international committee of scientists isresponsible for approving the names and chemical symbols ofthe elements. The names of the elements come from manysources. For example, some elements are named after impor-tant scientists (mendelevium, einsteinium), and others arenamed for geographical regions (germanium, californium).

The chemical symbol for each element usually consists ofone or two letters. The first letter in the symbol is always cap-italized, and the second letter, if there is one, is always writ-ten in lowercase. The chart below lists the patterns that thechemical symbols follow, and the Explore activity will helpyou investigate two of those patterns further.

Chapter 13332

L ook at the periodic table shownhere. How is it the same as the

periodic table you saw earlier? How is it different? Explain why it is importantfor scientific communication that the chemical symbols used are the same around the world.

Writing the Chemical Symbols

Draw a line down a sheet ofpaper to divide it into twocolumns. Look at the el-ements with atomic numbers1 through 10 on the periodictable. Write all the chemicalsymbols and names that fol-low one pattern in one col-umn on your paper and allchemical symbols and namesthat follow a second patternin the second column. Writea sentence describing eachpattern you found.

You can create your own well-rounded periodic table using

coins, washers, and buttons onpage 572 of the LabBook.

Pattern of chemical symbols

first letter of the name

first two letters of the name

first letter and third or laterletter of the name

letter(s) of a word other thanthe English name

first letter of root words thatstand for the atomic number(used for elements whose offi-cial names have not yet beenchosen)

Examples

S—sulfur

Ca—calcium

Mg—magnesium

Pb—lead (from the Latinplumbum, meaning “lead”)

Uun—ununnilium (uhn uhn NIL ee uhm) (foratomic number 110)


Rows Are Called Periods Each horizontal row of elements(from left to right) on the periodic table is called a period. Forexample, the row from lithium (Li) to neon (Ne) is Period 2.A row is called a period because the properties of elements ina row follow a repeating, or periodic, pattern as you moveacross each period. The physical and chemical properties ofelements, such as conductivity and the number of electronsin the outer level of atoms, change gradually from those of ametal to those of a nonmetal in each period. Therefore, el-ements at opposite ends of a period have very different prop-erties from one another, as shown in Figure 3.

Columns Are Called Groups Each column of elements(from top to bottom) on the periodic table is called a group.Elements in the same group often have similar chemical andphysical properties. For this reason, sometimes a group is alsocalled a family. You will learn more about each group in thenext section.

Figure 3 The elements in a row become lessmetallic from left to right.

1. Compare a period and a group on the periodic table.

2. How are the elements arranged in the modern periodictable?

3. Comparing Concepts Compare metals, nonmetals, andmetalloids in terms of their electrical conductivity.


To remember that a periodgoes from left to rightacross the periodic table,just think of reading a sen-tence. You read from left toright across the page untilyou come to a period.

REVIEW

Elements at the left end of aperiod, such as titanium, arevery metallic in their properties.

Elements farther to the right,like germanium, are lessmetallic in their properties.

Elements at the far right end ofa period, such as bromine, arenonmetallic in their properties.

VK Ca Sc Ti Cr Mn Fe Co19 20 21 22 23 24 25 26 27

Ni Cu Zn Ga Ge As Se Br Kr28 29 30 31 32 33 34 35 36

47.9Titanium

Ti22

72.6Germanium

Ge32

79.9Bromine

Br35


Chapter 13334

N E W T E R M Salkali metalsalkaline-earth metalshalogensnoble gases

O B J E CT I V E S! Explain why elements in a group

often have similar properties.! Describe the properties of the

elements in the groups of theperiodic table.

Section2 Grouping the Elements

You probably know a family with several members that looka lot alike. Or you may have a friend whose little brother orsister acts just like your friend. Members of a family often—but not always—have a similar appearance or behavior.Likewise, the elements in a family or group in the periodictable often—but not always—share similar properties. The prop-erties are similar because the atoms of the elements have thesame number of electrons in their outer energy level.

Groups 1 and 2: Very Reactive MetalsThe most reactive metals are the elements in Groups 1 and 2.What makes an element reactive? The answer has to do withelectrons in the outer energy level of atoms. Atoms will oftentake, give, or share electrons with other atoms in order to havea complete set of electrons in their outer energy level. Elementswhose atoms undergo such processes are reactive and combineto form compounds. Elements whose atoms need to take, give,or share only one or two electrons to have a filled outer leveltend to be very reactive.

The elements in Groups 1 and 2 are so reactive that theyare only found combined with other elements in nature. Tostudy the elements separately, the naturally occurringcompounds must first be broken apart through chemicalchanges.

Group 1: Alkali Metals

Alkali (AL kuh LIE) metals aresoft enough to be cut with aknife, as shown in Figure 4.The densities of the alkalimetals are so low that lithium,sodium, and potassium areactually less dense than water.

Group contains: MetalsElectrons in the outer level: 1Reactivity: Very reactiveOther shared properties: Soft; silver-colored; shiny;low density

Lithium

Sodium

Potassium

Rubidium

Cesium

Francium

Li

Na

K

Rb

Cs

Fr

3

11

19

37

55

87 Figure 4 Metals so soft thatthey can be cut with a knife?Welcome to the alkali metals.

Although the elementhydrogen appears above thealkali metals on the periodictable, it is not considered amember of Group 1. It willbe described separately at

the end of this section.


Alkali metals are the most reactive of the metals. This isbecause their atoms can easily give away the single electronin their outer level. For example, alkali metals react violentlywith water, as shown in Figure 5. Alkali metals are usuallystored in oil to prevent them from reacting with water andoxygen in the atmosphere.

The compounds formed from alkali metals have many uses.Sodium chloride (table salt) can be used to add flavor to yourfood. Sodium hydroxide can be used to unclog your drains.Potassium bromide is one of several potassium compoundsused in photography.

Group 2: Alkaline-earth Metals

Alkaline-earth metals are not as reactive as alkalimetals because it is more difficult for atoms to giveaway two electrons than to give away only onewhen joining with other atoms.

The alkaline-earth metal magnesium is oftenmixed with other metals to make low-densitymaterials used in airplanes. Compounds ofalkaline-earth metals also have many uses.For example, compounds of calcium arefound in cement, plaster, chalk, and evenyou, as shown in Figure 6.

Figure 5 As alkali metals react with water, they form hydrogen gas.

Group contains: MetalsElectrons in the outer level: 2Reactivity: Very reactive, but less reactive than alkalimetalsOther shared properties: Silver-colored; more densethan alkali metals

Beryllium

Magnesium

Calcium

Strontium

Barium

Radium

Be

Mg

Ca

Sr

Ba

Ra

4

12

20

38

56

88

Figure 6 Smile! Calcium, analkaline-earth metal, is animportant component of acompound that makes yourbones and teeth healthy.

335

Lithium PotassiumSodium


Groups 3–12: Transition Metals Groups 3–12 do not have individual names. Instead, thesegroups are described together under the name transition metals.

The atoms of transition metals do not give away their elec-trons as easily as atoms of the Group 1 and Group 2 metalsdo, making transition metals less reactive than the alkali metals and the alkaline-earth metals. The properties of thetransition metals vary widely, as shown in Figure 7.

Some transition metals, including the titanium in the artificial hip at right,are not very reactive. But others, suchas iron, are reactive. The iron in thesteel trowel above has reacted withoxygen to form rust.

Figure 7 Transition metals have a wide range of physical and chemical properties.

Chapter 13336

Group contains: MetalsElectrons in the outer level: 1 or 2Reactivity: Less reactive than alkaline-earth metalsOther shared properties: Shiny; good conductors of thermalenergy and electric current; higher densities and melting points(except for mercury) than elements in Groups 1 and 2

Self-CheckWhy are alkali metals more reactive than alkaline-earth metals? (See page 596 to check your answer.)

Rutherfordium

Molybdenum

Scandium

Yttrium

Lanthanum

Actinium

Titanium

Zirconium

Hafnium

Vanadium

Niobium

Tantalum

Dubnium

Chromium

Tungsten

Seaborgium

Manganese

Technetium

Rhenium

Bohrium

Iron

Ruthenium

Osmium

Hassium

Cobalt

Rhodium

Iridium

Meitnerium

VSc

Y

La

Ac

Ti

Zr

H f

Rf

Nb

Ta

Db

Cr

Mo

W

Sg

Mn

Re

Bh

IrOs

Fe

Ru

Hs

Co

Rh

Mt

Tc

21

39

57

89

22

40

72

104

23

41

73

105

24

42

74

106

25

43

75

107

26

44

76 77

108

27

45

109

Nickel Copper Zinc

Palladium Silver Cadmium

Ununnilium Unununium

Platinum Gold Mercury

Ununbium

Pd Ag Cd

Pt

Uun Uuu

Au Hg

Ni Cu Zn

Uub

28 29 30

46 47 48

78 79 80

110 111 112

Mercury is used in thermometers because,unlike the other transition metals, it is inthe liquid state at room temperature.

Many transition metals aresilver-colored—but not all!This gold ring proves it!


Lanthanides and Actinides Some transitionmetals from Periods 6 and 7 are placed at the bot-tom of the periodic table to keep the table frombeing too wide. The properties of the elements ineach row tend to be very similar.

Elements in the first row are calledlanthanides because they follow thetransition metal lanthanum. The lan-thanides are shiny, reactive metals.Some of these elements are used tomake different types of steel. An impor-tant use of a compound of one lan-thanide element is shown in Figure 8.

Elements in the second row arecalled actinides because they follow thetransition metal actinium. All atoms ofactinides are radioactive, which meansthey are unstable. The atoms of aradioactive element can change intoatoms of a different element. Elementslisted after plutonium, element 94, donot occur in nature but are insteadproduced in laboratories. You mighthave one of these elements in yourhome. Very small amounts of americ-ium (AM uhr ISH ee uhm), element 95,are used in some smoke detectors.


1. What are two properties of the alkali metals?

2. What causes the properties of elements in a group to besimilar?

3. Applying Concepts Why are neither the alkali metalsnor the alkaline-earth metals found uncombined innature?

REVIEW

138.9Lanthanum

La57

(227.0)Actinium

Ac89

58

90

59

91

60

92

61

93

62

94

Ce

Th

Pr

Pa

Nd

U

Pm

Np

Sm

Pu

Eu

Am

Gd

Cm

Tb

Bk

Dy

Cf

Ho

Es

Er

Fm

Tm

Md

Yb

No

Lu

Lr

63

95

64

96

65

97

66

98

67

99

68

100

69

101

70

102

71

103

Lanthanides

Actinides

Figure 8 Seeing red? The color red appears on acomputer monitor because of a compound formedfrom europium that coats the back of the screen.


Groups 13–16:Groups That Include MetalloidsMoving from Group 13 across to Group 16, the elements shiftfrom metals to nonmetals. Along the way, you find the met-alloids. These elements have some properties of metals andsome properties of nonmetals.

Group 13: Boron Group

The most common element from Group 13 is alu-minum. In fact, aluminum is the most abundantmetal in Earth’s crust. Until the 1880s, it was con-sidered a precious metal because the process usedto produce pure aluminum was very expensive. Infact, aluminum was even more valuable than gold,as shown in Figure 9.

Today, the process is not as difficult or expen-sive. Aluminum is now an important metal usedin making lightweight automobile parts and air-craft, as well as foil, cans, and wires.

Group 14: Carbon Group

The metalloids silicon and germanium are used tomake computer chips. The metal tin is usefulbecause it is not very reactive. A tin can is reallymade of steel coated with tin. The tin is less reac-tive than the steel, and it keeps the steel from rusting.

Chapter 13338

Group contains: One metalloid and four metalsElectrons in the outer level: 3Reactivity: ReactiveOther shared properties: Solid at room temperature

Gallium

Aluminum

Boron

Indium

Thallium

In

Tl

Ga

Al

B

31

13

5

49

81

Group contains: One nonmetal, two metalloids, andtwo metalsElectrons in the outer level: 4Reactivity: Varies among the elementsOther shared properties: Solid at room temperature

Germanium

Silicon

Carbon

Tin

Lead

Sn

Pb

Ge

Si

C

32

14

6

50

82

Figure 9 During the 1850s and 1860s, EmperorNapoleon III of France, nephew of NapoleonBonaparte, used aluminum dinnerware becausealuminum was more valuable than gold!

environmentalscienceC O N N E C T I O N

Recycling aluminum uses lessenergy than obtaining alumi-num in the first place. Alumi-num must be separated frombauxite, a mixture containingnaturally occurring compoundsof aluminum. Twenty timesmore electrical energy is re-quired to separate aluminumfrom bauxite than to recycleused aluminum.


The nonmetal carbon can be found uncombined innature, as shown in Figure 10. Carbon forms a wide varietyof compounds. Some of these compounds, including pro-teins, fats, and carbohydrates, are essential to life on Earth.

Group 15: Nitrogen Group

Nitrogen, which is a gas at room temperature,makes up about 80 percent of the air you breathe.Nitrogen removed from air is reacted with hydro-gen to make ammonia for fertilizers.

Although nitrogen is unreactive, phosphorus isextremely reactive, as shown in Figure 11. In fact,phosphorus is only found combined with otherelements in nature.

Group 16: Oxygen Group

Oxygen makes up about 20 percent of air. Oxygenis necessary for substances to burn, such as thechemicals on the match in Figure 11. Sulfur,another common member of Group 16, can befound as a yellow solid in nature. The principaluse of sulfur is to make sulfuric acid, the mostwidely used compound in the chemical industry.

Figure 10 Diamonds and soot have very differentproperties, yet both are natural forms of carbon.

Diamond is the hardest materialknown. It is used as a jeweland on cutting tools such assaws, drills, and files.

Figure 11Simply striking a matchon the side of this boxcauses chemicals onthe match to reactwith phosphorus on the box andbegin to burn.


Group contains: Two nonmetals, two metalloids, andone metalElectrons in the outer level: 5Reactivity: Varies among the elementsOther shared properties: All but nitrogen are solid atroom temperature.

Arsenic

Phosphorus

Nitrogen

Antimony

Bismuth

Sb

Bi

As

P

N

33

15

7

51

83

Group contains: Three nonmetals, one metalloid, andone metalElectrons in the outer level: 6Reactivity: ReactiveOther shared properties: All but oxygen are solid atroom temperature.

Selenium

Sulfur

Oxygen

Tellurium

Polonium

Te

Po

Se

S

O

34

16

8

52

84

A particle of carbon shaped like a soccer ball? You’ll get a kick out of reading about buckyballs on page 347.

Soot—formed from burning oil, coal, and wood—is used as a pigment in paints and crayons.


Groups 17 and 18: Nonmetals OnlyThe elements in Groups 17 and 18 are nonmetals. The el-ements in Group 17 are the most reactive nonmetals, but theelements in Group 18 are the least reactive nonmetals. In fact,the elements in Group 18 normally won’t react at all withother elements.

Group 17: Halogens

Halogens are very reactive nonmetals because theiratoms need to gain only one electron to have acomplete outer level. The atoms of halogens com-bine readily with other atoms, especially metals,to gain that missing electron.

Although the chemical properties of the halo-gens are similar, the physical properties are quitedifferent, as shown in Figure 12.

Both chlorine and iodine are used as disinfec-tants. Chlorine is used to treat water, while iodinemixed with alcohol is used in hospitals.

Group 18: Noble Gases

Noble gases are unreactive nonmetals. Because theatoms of the elements in this group have a com-plete set of electrons in their outer level, they donot need to lose or gain any electrons. Therefore,they do not react with other elements under nor-mal conditions.

All of the noble gases are found in Earth’satmosphere in small amounts. Argon, the mostabundant noble gas in the atmosphere, makes upalmost 1 percent of the atmosphere.

Figure 12 Physical propertiesof some halogens at roomtemperature are shown here.

340

Group contains: NonmetalsElectrons in the outer level: 7Reactivity: Very reactiveOther shared properties: Poor conductors of electriccurrent; react violently with alkali metals to form salts;never found uncombined in nature

T-P01-039-

Bromine

Chlorine

Fluorine

Iodine

Astatine

I

At

Br

Cl

F

35

17

9

53

85

Group contains: NonmetalsElectrons in the outer level: 8 (2 for helium)Reactivity: UnreactiveOther shared properties: Colorless, odorless gases atroom temperature

Krypton

Argon

Neon

Helium

Xenon

Radon

Xe

Rn

Kr

Ar

Ne

He

36

18

10

2

54

86

The term noble gasesdescribes the nonreactivityof these elements. Just asnobles, such as kings andqueens, did not often mixwith common people, thenoble gases do not normallyreact with other elements.

Chapter 13

Chlorine is ayellowishgreen gas.

Bromine is adark red liquid.

Iodine is a darkgray solid.


The nonreactivity of the noble gasesmakes them useful. Ordinary light bulbslast longer when filled with argon thanthey would if filled with a reactive gas.Because argon is unreactive, it does notreact with the metal filament in the lightbulb even when the filament gets hot.The low density of helium causes blimpsand weather balloons to float, and itsnonreactivity makes helium safer to usethan hydrogen. One popular use of noblegases that does not rely on their nonre-activity is shown in Figure 13.

Hydrogen Stands Apart

The properties of hydrogen do not match the properties ofany single group, so hydrogen is set apart from the other el-ements in the table.

Hydrogen is placed above Group 1 in the periodic tablebecause atoms of the alkali metals also have only one elec-tron in their outer level. Atoms of hydrogen, like atoms ofalkali metals, can give away one electron when joining withother atoms. However, hydrogen’s physical properties are morelike the properties of nonmetals than of metals. As you cansee, hydrogen really is in a group of its own.

Hydrogen is the most abundant element in the universe.Hydrogen’s reactive nature makes it useful as a fuel in rockets,as shown in Figure 14.

Figure 14 Hydrogen is a reactive nonmetal. As hydrogenburns, it joins with oxygen, andthe hot water vapor that formspushes the rocket up.


1. In which group are the unreactive nonmetals found?

2. What are two properties of the halogens?

3. Making Predictions In the future, a new noble gas maybe synthesized. Predict its atomic number and properties.

4. Comparing Concepts Compare the element hydrogenwith the alkali metal sodium.

Figure 13 Besides neon, other noble gases are oftenused in “neon” lights.

Electrons in the outer level: 1Reactivity: ReactiveOther properties: Colorless, odorless gas at room tem-perature; low density; reacts explosively with oxygen

REVIEW

HydrogenH1

Argon producesa lavender color.

Xenon producesa blue color.

Neon produces anorange-red color.

Helium producesa yellow color.


Chapter Highlights

Chapter 13342

SECTION 1

Vocabularyperiodic (p. 326)periodic law (p. 327)period (p. 333)group (p. 333)

Section Notes• Mendeleev developed the

first periodic table. Hearranged elements in order of increasing atomic mass. The properties of elementsrepeated in an orderly pat-tern, allowing Mendeleev to predict properties for el-ements that had not yet beendiscovered.

• Moseley rearranged the elements in order of increas-ing atomic number.

• The periodic law states thatthe chemical and physicalproperties of elements areperiodic functions of theiratomic numbers.

• Elements in the periodictable are divided into metals, metalloids, andnonmetals.

• Each element has a chemicalsymbol that is recognizedaround the world.

• A horizontal row of elementsis called a period. The el-ements gradually changefrom metallic to nonmetallicfrom left to right across eachperiod.

• A vertical column of el-ements is called a group or family. Elements in agroup usually have similarproperties.

LabsCreate a Periodic Table (p. 572)

Skills CheckVisual UnderstandingPERIODIC TABLE OF THE ELEMENTS Scientistsrely on the periodic table as a resource for alarge amount of information. Review theperiodic table on pages 328–329. Pay closeattention to the labels and the key; they will help you understand the informationpresented in the table.

CLASSES OF ELEMENTS Identifying an elementas a metal, nonmetal, or metalloid gives you abetter idea of the properties of that element.Review the figures on pages 330–331 tounderstand how to use thezigzag line on the periodictable to identify the classesof elements and to reviewthe properties of elementsin each category.


343The Periodic Table

SECTION 2

Vocabularyalkali metals (p. 334)alkaline-earth metals (p. 335)halogens (p. 340)noble gases (p. 340)

Section Notes• The alkali metals (Group 1)

are the most reactive metals.Atoms of the alkali metalshave one electron in theirouter level.

• The alkaline-earth metals(Group 2) are less reactivethan the alkali metals. Atomsof the alkaline-earth metalshave two electrons in theirouter level.

• The transition metals(Groups 3–12) include mostof the well-known metals aswell as the lanthanides andactinides located below theperiodic table.

• Groups 13–16 contain themetalloids along with somemetals and nonmetals. Theatoms of the elements ineach of these groups havethe same number of elec-trons in their outer level.

• The halogens (Group 17) arevery reactive nonmetals.Atoms of the halogens haveseven electrons in their outerlevel.

• The noble gases (Group 18)are unreactive nonmetals.Atoms of the noble gaseshave a complete set of elec-trons in their outer level.

• Hydrogen is set off by itselfbecause its properties do notmatch the properties of anyone group.

Visit the National Science Teachers Association on-line Website for Internet resources related to this chapter. Just type inthe sciLINKS number for more information about the topic:

TOPIC: The Periodic Table sciLINKS NUMBER: HSTP280TOPIC: Metals sciLINKS NUMBER: HSTP285TOPIC: Metalloids sciLINKS NUMBER: HSTP290TOPIC: Nonmetals sciLINKS NUMBER: HSTP295TOPIC: Buckminster Fuller and the Buckyball sciLINKS NUMBER: HSTP300

Visit the HRW Web site for a variety oflearning tools related to this chapter. Just type in the keyword:

KEYWORD: HSTPRT

GO TO: go.hrw.com GO TO: www.scilinks.org


Chapter ReviewUSING VOCABULARY

Complete the following sentences by choos-ing the appropriate term from each pair ofterms listed below.

1. Elements in the same vertical column inthe periodic table belong to the same ? . (group or period)

2. Elements in the same horizontal row inthe periodic table belong to the same ? . (group or period)

3. The most reactive metals are ? . (alkali metals or alkaline-earth metals)

4. Elements that are unreactive are called ? . (noble gases or halogens)

UNDERSTANDING CONCEPTS

Multiple Choice

5. An element that is a very reactive gas ismost likely a member of thea. noble gases. c. halogens.b. alkali metals. d. actinides.

6. Which statement is true?a. Alkali metals are generally found in

their uncombined form.b. Alkali metals are Group 1 elements.c. Alkali metals should be stored under

water.d. Alkali metals are unreactive.

7. Which statement about the periodic tableis false?a. There are more metals than nonmetals.b. The metalloids are located in Groups 13

through 16.c. The elements at the far left of the table

are nonmetals.d. Elements are arranged by increasing

atomic number.

8. One property of most nonmetals is thatthey area. shiny.b. poor conductors of electric current.c. flattened when hit with a hammer.d. solids at room temperature.

9. Which is a true statement about elements?a. Every element occurs naturally.b. All elements are found in their uncom-

bined form in nature.c. Each element has a unique atomic

number.d. All of the elements exist in approxi-

mately equal quantities.

10. Which is NOT found on the periodictable?a. The atomic number of each elementb. The symbol of each elementc. The density of each elementd. The atomic mass of each element

Short Answer

11. Why was Mendeleev’s periodic table useful?

12. How is Moseley’s basis for arranging theelements different from Mendeleev’s?

13. How is the periodic table like a calendar?

14. Describe the location of metals, metal-loids, and nonmetals on the periodictable.

Chapter 13344Copyright © by Holt, Rinehart and Winston. All rights reserved.

Concept Mapping

15. Use the followingterms to create aconcept map: peri-odic table, elements,groups, periods,metals, nonmetals,metalloids.

CRITICAL THINKING AND PROBLEM SOLVING

16. When an element with 115 protons in itsnucleus is synthesized, will it be a metal,a nonmetal, or a metalloid? Explain.

17. Look at Mendeleev’s periodic table inFigure 2. Why was Mendeleev not able tomake any predictions about the noble gaselements?

18. Your classmate offers to give you a pieceof sodium he found while hiking. What isyour response? Explain.

19. Determine the identity of each elementdescribed below:a. This metal is very reactive, has proper-

ties similar to magnesium, and is in thesame period as bromine.

b. This nonmetal is in the same group as lead.

c. This metal is the most reactive metal inits period and cannot be found uncom-bined in nature. Each atom of the el-ement contains 19 protons.

MATH IN SCIENCE

20. The chart below shows the percentages ofelements in the Earth’s crust.

Excluding the “Other” category, what per-centage of the Earth’s crust isa. alkali metals?b. alkaline-earth metals?

INTERPRETING GRAPHICS

21. Study the diagram below to determine thepattern of the images. Predict the missingimage, and draw it. Identify which prop-erties are periodic and which propertiesare shared within a group.

Take a minute to review your answersto the ScienceLog questions on page325. Have your answers changed? Ifnecessary, revise your answers based onwhat you have learned since you beganthis chapter.

?fpo

47-A

46.6% O

27.7% Si

2.0% Mg

2.8% Na

3.6% Ca

5.0% Fe

8.1% Al

2.6% K

1.6% Other

345The Periodic TableCopyright © by Holt, Rinehart and Winston. All rights reserved.

346

The Science of Fireworks

What do the space shuttle and theFourth of July have in common? Thesame scientific principles that help

scientists launch a space shuttle also helppyrotechnicians create spectacular fireworksshows. The word pyrotechnics comes from theGreek words for “fire art.” Explosive and daz-zling, a fireworks display is both a science andan art.

An Ancient HistoryMore than 1,000 years ago, Chinese civilizationsmade black powder, the original gunpowderused in pyrotechnics. They used the powder toset off firecrackers and primitive missiles. Blackpowder is still used today to launch fireworksinto the air and to give fireworks an explosivecharge. Even the ingredients—saltpeter (potas-sium nitrate), charcoal, and sulfur—haven’tchanged since ancient times.

Snap, Crackle, Pop!The shells of fireworks contain the ingredientsthat create the explosions. Inside the shells,black powder and other chemicals are packedin layers. When ignited, one layer may cause abright burst of light while a second layer pro-duces a loud booming sound. The shell’s shapeaffects the shape of the explosion. Cylindricalshells produce a trail of lights that looks like anumbrella. Round shells produce a star-burstpattern of lights.

The color and sound of fireworks dependon the chemicals used. To create colors, chemi-cals like strontium (for red), magnesium (forwhite), and copper (for blue) can be mixedwith the gunpowder.

Explosion in the SkyFireworks are launched from metal, plastic, orcardboard tubes. Black powder at the bottomof the shell explodes and shoots the shell intothe sky. A fuse begins to burn when the shell islaunched. Seconds later, when the explosivechemicals are high in the air, the burning fuselights another charge of black powder. Thisignites the rest of the ingredients in the shell,causing an explosion that lights up the sky!

Bang for Your Buck! The fireworks used during New Year’s Eveand Fourth of July celebrations can cost any-where from $200 to $2,000 apiece. Count thenumber of explosions at the next fireworksshow you see. If each of the fireworks cost just$200 to produce, how much would the fire-works for the entire show cost?

" Cutaway view of a typicalfirework. Each shell createsa different type of display.

Quick-burning fuse

Time-delay fuse

Light-burst mixture

Fuse

Sound-burst mixture

Black-powder propellant


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B U C K Y B A L LSResearchers are scrambling for the ball—thebuckyball, that is. This special form of carbonhas 60 carbon atoms linked together in a shapemuch like a soccer ball. Scientists are having afield day trying to find new uses for thisunusual molecule.

The Starting LineupNamed for architect Buckminster Fuller, bucky-balls resemble the geodesic domes that arecharacteristic of the architect’s work. Excitementover buckyballs began in 1985 when scientistsprojected light from a laser onto a piece ofgraphite. In the soot that remained, researchersfound a completely new kind of molecule!Buckyballs are also found in the soot from acandle flame. Some scientists claim to havedetected buckyballs in outer space. In fact, one

! The buckyball, short for buckminster-fullerene, was named after architectBuckminster Fuller.

Potassium atom trappedinside buckyball

Carbonatoms

Bond

hypothesis suggests that buckyballs might be atthe center of the condensing clouds of gas, dust,and debris that form galaxies.

The Game PlanEver since buckyballs were discovered, chemistshave been busy trying to identify the molecules’properties. One interesting property is that sub-stances can be trapped inside a buckyball. Abuckyball can act like a cage that surroundssmaller substances, such as individual atoms.Buckyballs also appear to be both slippery andstrong. They can be opened to insert materials,and they can even link together in tubes.

How can buckyballs be used? They mayhave a variety of uses, from carrying messagesthrough atom-sized wires in computer chips todelivering medicines right where the bodyneeds them. Making tough plastics and cuttingtools are uses that are also under investigation.With so many possibilities,scientists expect to geta kick out of bucky-balls for sometime!

The Kickoff" A soccer ballis a great modelfor a buckyball.On the model, theplaces where threeseams meet correspondto the carbon atoms on a buckyball. What represents the bonds betweencarbon atoms? Does your soccer-ball modelhave space for all 60 carbon atoms? You’ll haveto count and see for yourself.


timeline unit the atom · 2014-09-11 · his results demonstrated that elements combine in specific...

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