Ch. 3: AtomsSec. 1: From Philosophical Idea to Scientific Theory

History of the atom

• Greeks were first to try to explain chemical reactions

• 400 BC: thought all matter composed of:– Fire– Earth– Water– Air

• Democritus first used word “atomos”, meaning indivisible

Nothing exists but atoms and empty space; everything else is opinion. (Demokritos)

Next 2000 years

• Alchemy gave basis for modern chemistry

• Two ideas– Turning cheap metals into gold and

silver– Finding the “elixir of life”

• Did identify elements and learn how to prepare acids

• Very secretive

Robert Boyle (1627-1691)

• Defined element: any substance that cannot be broken down into simpler substances

• Felt science supported religion• End of four element theory

Robert Boyle (1627-1691)

(Not necessary to copy what is in italics)

“I now mean by elements, as these chymists that speak plainest do by their principles, certain primitive and simple or perfectly unmingled bodies, which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved.”

Antoine Lavoisier

• Father of chemistry• Lost his head during the French

Revolution• Devised metric system in 1771• Responsible for naming and

listing elements of the day

• Lavoisier’s wife played a big role in his work, unusual for the times.

The Elements• 110 known elements• 88 (?) occur naturally

The elements form a plethora of compounds, just as 26 letters of the alphabet make a seemingly endless number of words.

Elements in Earth’s Crust, Oceans and Atmosphere*

• These eight elements account for approximately 98.5% of the total mass of the earth's crust.

oxygen (46.6%) calcium (3.6%)

silicon (27.7%) sodium (2.8%)

aluminum (8.1%) potassium (2.6%)

iron (5.0%) magnesium (2.1%)

*Know the top two.

Elements in the Human Body*

• Oxygen (65%) • Carbon (18%) • Hydrogen (10%) • Nitrogen (3%) • Calcium (1.5%) • Phosphorus (1.0%) • Potassium (0.35%) • Sulfur (0.25%) • Sodium (0.15%) • Magnesium (0.05%) • Copper, Zinc, Selenium, Molybdenum, Fluorine,

Chlorine, Iodine, Manganese, Cobalt, Iron (0.70%) • Lithium, Strontium, Aluminum, Silicon, Lead,

Vanadium, Arsenic, Bromine (trace amounts)

*Know the top four.

Symbols for the Elements

• Sources of element names come from the Greek, Latin & German languages

• Symbols simplify writing by using abbreviations– 1st letter always capitalized (C, F)– 2nd letter always small case (Zn, Na)

Symbols from Latin Names

Element Symbol Latin name

Copper Cu cuprum

Gold Au aurum

Lead Pb plumbum

Mercury Hghydrargyrum

Potassium K kalium

Silver Ag argentum

Sodium Na natrium

Tin Sn stannum

Symbols and Compounds

• Chemical formulas represent compounds– Symbols indicate the elements present– Subscripts show the relative numbers

of atoms of each element

• KCl: potassium and chlorine, 1 of each

• CCl4: One atom of carbon and 4 atoms of chlorine

• C6H12O6

Atomic TheoryStates that all matter is composed of discrete units called atoms

Democritus

• Matter consists of basic particles that cannot be divided (atoms)

• “Atomos” means uncut or indivisible

18th century Observations

• Natural materials are mixtures of pure substances

• Pure substances can be either elements or compounds (Water was thought to be an element until electrical current was used to separate the hydrogen and oxygen

• Law of Conservation of Mass– Mass is neither created nor destroyed

in a chemical or physical reaction

18th century Observations, cont.• Law of Constant Composition

(Proust)– A given compound always has the

same composition, regardless of where it comes from

• Law of Definite Proportions– When elements combine to form

chemical compounds, they unite in definite proportions by mass

• In 18 grams of water, there are 2 g of hydrogen and 16 g of oxygen

• In 72 g of water, there are 8 g of hydrogen and 64 g of oxygen

• Ratio of hydrogen to oxygen is always 1:8

John Dalton (1766–1844)

• Englishman• Was a school teacher at age 12• Extremely interested in meteorology• Collected air samples all over Europe• Made daily weather observations for 57

years (Jeff Lyons would have loved him!)• Wondered about the composition of the

air, which led to his atomic theory

Dalton’s Atomic Theory (1808-1810)

1. All elements are made of tiny particles called atoms

2. All atoms of an element are identical

3. All atoms of a given element are different from those of any other element

4. Atoms of 1 element can combine with atoms of other elements to form compounds.

5. Atoms are indivisible, and are neither created or destroyed.

Reactions simply change the way atoms are grouped together.

6. A given compound contains the same relative numbers and types of atoms

• We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen. John Dalton, A New System of Chemical Philosophy, 1808)

Dalton’s Symbols for the Elements

• John Dalton was the first person to use ball-and-stick models to represent molecules. He asked his friend Peter Ewart, a Manchester engineer, to make a series of them for him in about 1810. These wooden balls were made to demonstrate Dalton's theory of how atoms in solids, liquids and gases are arranged. He used these models in his lectures and for other demonstrations, but failed to publish details of them until 1842, two years before his death.

12 g C16g O

12 g C32 g O

For the same amount of carbon, the ratio of oxygen in the 2 compounds is 1:2.

Dalton's Model

• Not accepted at first• Law of Multiple Proportions

– Relates to two compounds made by the same elements

– Carbon dioxide has twice the mass of oxygen as carbon monoxide

Changes to Dalton’s Theory

• Took until early 1900’s– Atom was divisible (nuclear physics)– Isotopes existed

• We still use his model (with some revisions) today

Modern Atomic Theory

Changes to Dalton’s theory:• Atoms of an element are nearly

identical, but may have different masses

• Atoms are not indivisible, but are composed of subatomic particles

• Matter is neither created nor destroyed in ordinary chemical reactions

Pop Quiz

1. What are the 2 most abundant elements in the human body?

2. Write 3 postulates of Dalton’s atomic theory.

3. Describe the Law of Multiple Proportions

Pop Quiz1. Oxygen and carbon

2. • Elements are made of tiny particles called atoms• All atoms of an element are identical• All atoms of a given element are different from those of any other

element• Atoms of 1 element can combine with atoms of other elements to form

compounds. • Atoms are indivisible, and are neither created or destroyed. • Reactions simply change the way atoms are grouped

together.• A given compound contains the same relative numbers and types of

atoms

3.

Sec. 2: The Structure of the Atom

• J. J. Thomson (1897)– Discovered the electron– Plum Pudding Model

• Ernest Rutherford– Showed existence of a nucleus

How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron.

observation

ray properties are independent of the cathode material

hypothesis

cathode ray stuff is a component of all materials

How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron.

observation

cathode rays bend near magnets

hypothesis

magnets bend the paths of moving charged particles; maybe cathode rays are streams of moving charged particles

How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron.

observationrays bend towards a positively charged plate.rays impart a negative charge to objects they strike.

hypothesiscathode rays are streams of negative charges

How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron.

observationCathode rays don't bend around small obstacles,cast sharp shadows,can turn paddlewheels placed in their path, and travel in straight lines

hypothesiscathode rays behave like streams of particles (subatomic particles means smaller than atoms)

Positive fieldCathode (-)

Cathode (-)

Gas at low pressure Paddle Wheel

Anode (+)

Paddle Wheel Track

Cathode Ray Electron Beam

Gas at low pressure

Cathode Ray Electron Beam

Anode (+)

Metal DiskElectric Source

Negative field

Thomson’s Conclusions• Cathode rays are negatively

charged subatomic particles• Later named electrons• Also determine the ratio of the

charge of electron to its mass to be approx. 1 x 107 (BIG number!)

Plum Pudding Model

• Atom has a spherical cloud of positive charge (pudding) with electrons (plums) scattered within it that make the atom neutral

Robert Millikan

• Discovered the charge of an electron

• From that information and the charge/mass ratio, the mass of an electron was calculated to be about 1/2000 the mass of a hydrogen atom

Ernest Rutherford's scattering experiment hypothesis:

• If the plum pudding model of the atom is correct, atoms have no concentration of mass or charge (atoms are 'soft' targets)

Experiment to test hypothesis:

– fire massive alpha particles at the atoms in thin metal foil

– alpha particles should pass like bullets straight through soft plum pudding atoms

observation:

a few alpha particles ricocheted!

http://micro.magnet.fsu.edu/electromag/java/rutherford/

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

New hypotheses:

– all of the positive charge and nearly all of the mass of the atom is concentrated in a tiny, incredibly dense 'nucleus', about 10-14 m in diameter

– electrons roam empty space about 10-10 m across, around the nucleus

Nucleus (Protons & Neutrons)

Orbital (Space Occupied by Electrons)

Proton

Neutron

If nucleus = basketballThe circle = outside edge of atom

Discovery of the neutron

• James Chadwick (1932)• Helped explain the discrepancy

between the charge and mass of an atom

Subatomic Particle Comparison

Subatomic Particle

Relative charge

Relative mass

Location

Proton +1 1 Nucleus

Neutron 0 1 Nucleus

Electron -1 .0005 Orbital

Nuclear Forces• Like charges repel, but there can be

many positive protons in a nucleus• Held by very strong but very short-ranged

attractions• Forces are proton-proton, proton-neutron,

and neutron-neutron

Sizes of Atoms

• Atomic radii range from 40-270 pm– 1pm = 10-12m (Compare to size of 1

cm compared to 1000 km, or 600 mi.)• Nuclear radii are about 0.001 pm

and have a density of 2x108 metric tons/cm3

Atomic Number

• Counts the number of protons

in an atom

• Protons determine the element’s identity

Atomic Number on the Periodic Table

11

Na

Atomic Number

Symbol

All atoms of an element have the same number of protons

11

Na11 protons

Sodium

Learning Check AT 1

State the number of protons for atoms of each of the following:

A. Nitrogen

1) 5 protons 2) 7 protons 3) 14 protons

B. Sulfur

1) 32 protons 2) 16 protons 3) 6 protons

C. Barium

1) 137 protons 2) 81 protons 3) 56 protons

Solution AT 1

State the number of protons for atoms of each of the following:

A. Nitrogen

2) 7 protons

B. Sulfur

2) 16 protons

C. Barium

3) 56 protons

Number of Electrons

An atom is neutral The net charge is zeroNumber of protons = Number of electronsIn a neutral atom, the atomic number gives us

the number of electrons as well It is the electrons that determine the chemical

properties of an atom.

Isotopes

• Isotopes are atoms of the same element that have different masses.

• The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons.

• Most of the elements consist of mixtures of isotopes.

Mass Number

Counts the number of protons and neutrons in an atom

Designating isotopes

• Hyphen notation: The mass number is written with a hyphen after the name of the element.

• uranium-235

• Nuclear symbol: The superscript indicates the mass number and the subscript indicates the atomic number.

235 92 U

Isotope Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11

AZ X

More Atomic Symbols

16 31 65 O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Learning Check AT 2

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#P _______ _______ _______

#N _______ _______ _______

#E _______ _______ _______

Solution AT 2

12C 13C 14C 6 6 6

#P __6___ _ 6___ ___6___

#N __6___ _ _7___ ___8___

#E __6___ _ 6___ ___6___

Learning Check AT 3

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30 2) 35 3) 65

B. Number of neutrons in the zinc atom

1) 30 2) 35 3) 65

C. What is the mass number of a zinc isotope

with 37 neutrons?

1) 37 2) 65 3) 67

Solution AT 3

An atom of zinc has a mass number of 65.

A. Number of protons in the zinc atom

1) 30

B. Number of neutrons in the zinc atom

2) 35

C. What is the mass number of a zinc isotope

with 37 neutrons?

3) 67

Learning Check AT 4

Write the atomic symbols for atoms with the following:

A. 8 p+, 8 n, 8 e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Solution AT 4

16OA. 8 p+, 8 n, 8 e- 8

B. 17p+, 20n, 17e- 37Cl 17

C. 47p+, 60 n, 47 e- 107Ag 47

Learning Check AT 5

An atom has 14 protons and 20 neutrons.A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is1) 14 2) 16 3) 34

C. The element is1) Si 2) Ca 3) Se

D. Another isotope of this element is

1) 34X 2) 34X 3) 36X 16 14 14

Solution AT 5

An atom has 14 protons and 20 neutrons.A. It has atomic number

1) 14

B. It has a mass number of3) 34

C. The element is1) Si

D. Another isotope of this element would be

3) 36X 14