4.3 Modern Atomic Theory

Reading StrategySequencing Copy the flowchart. After youread, complete the description of how a gainor loss of energy affects atoms.

Key ConceptsWhat can happen toelectrons when atomsgain or lose energy?

What model do scientistsuse to describe howelectrons behave in atoms?

What is the most stableconfiguration of electronsin an atom?

Vocabulary◆ energy levels◆ electron cloud◆ orbital◆ electron

configuration◆ ground state

Electrons and Energy Levels

Emitsenergy

Excitedstate

a. ? b. ?

Have you ever wondered what produces the different colors ina fireworks display? Why does one explosion produce red lightand another explosion produce green light? The people who makefireworks know that certain compounds will produce certaincolors of light when they are heated. For example, compoundscontaining the element strontium produce red light when theyare heated. Compounds containing barium produce green light.

You have seen two things that can happen when atoms absorbenergy—an increase in kinetic energy or a phase change. But thereis another possibility. The energy may be temporarily absorbed bythe atom and then emitted as light. The colors in a fireworks dis-play are a clue to how electrons are arranged in atoms.

Bohr’s Model of the AtomYou may have seen diagrams of an atom that look like a solarsystem with planets revolving around a sun. These diagrams arebased on a model of the atom that was developed by Niels Bohr(1885–1962), a Danish physicist who worked for a while withRutherford. Bohr agreed with Rutherford’s model of a nucleussurrounded by a large volume of space. But Bohr’s model didsomething that Rutherford’s model did not do. It focused on theelectrons. A description of the arrangement of electrons in anatom is the centerpiece of the modern atomic model.

Figure 13 Fireworks are often displayed above theLincoln Memorial in Washington, D.C. The red lightwas produced by a strontium compound.

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FOCUS

Objectives4.3.1 Describe Bohr’s model of the

atom and the evidence forenergy levels.

4.3.2 Explain how the electroncloud model represents thebehavior and locations ofelectrons in atoms.

4.3.3 Distinguish the groundstate from excited statesof an atom based onelectron configurations.

Build VocabularyLINCS Have students use the LINCSstrategy to learn the terms energy levels,electron cloud, orbital, electron configu-ration, and ground state. In LINCSexercises, students List what they knowabout each term, Imagine a picture thatdescribes the term, Note a reminding“sound-alike” word, Connect the termsto the sound-alike word by making upa short story, and then perform a briefSelf-test.

Reading Strategya. Electron moves to higher energy level.b. Electron moves to lower energy level.

INSTRUCT

Bohr’s Model ofthe AtomBuild Reading LiteracyRelate Text and Visuals Refer topage 190D in Chapter 7, whichprovides the guidelines for relatingtext and visuals.

Have students read Bohr’s Model of theAtom on pp. 113–116. Then, havestudents examine the diagram of Bohr’smodel in the time line on p. 115. Ask,What do the circles around the nucleusrepresent? (They represent energy levels.)Visual

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Atomic Structure 113

Print• Laboratory Manual, Investigations 4A

and 4B• Reading and Study Workbook With

Math Support, Section 4.3• Transparencies, Section 4.3

Technology• Interactive Textbook, Section 4.3• Presentation Pro CD-ROM, Section 4.3• Go Online, NSTA SciLinks, Energy levels

Section Resources

Section 4.3

Energy Levels In Bohr’s model, electrons move with constantspeed in fixed orbits around the nucleus, like planets around a sun.Each electron in an atom has a specific amount of energy. If an atomgains or loses energy, the energy of an electron can change. The possi-ble energies that electrons in an atom can have are called energy levels.

To understand energy levels, picture them as steps in a staircase.As you move up or down the staircase, you can measure how yourposition changes by counting the number of steps you take. You mighttake one step up, or you might jump two steps down. Whether you aregoing up or down, you can move only in whole-step increments. Justas you cannot stand between steps on a staircase, an electron cannotexist between energy levels.

1803 John Daltonpictures atoms astiny, indestructibleparticles, with nointernal structure.

1897 J. J. Thomson, a Britishscientist, discovers the electron,leading to his “plum-pudding”model. He pictures electronsembedded in a sphere ofpositive electric charge.

1904 HantaroNagaoka, a Japanesephysicist, suggests thatan atom has a centralnucleus. Electronsmove in orbits like therings around Saturn.

Models of the AtomThe development of scientific ideas onthe structure of atoms has passed severalkey milestones during the last 200 years.

1911 New ZealanderErnest Rutherfordstates that an atom hasa dense, positivelycharged nucleus.Electrons moverandomly in the spacearound the nucleus.

Tiny,solid sphereDALTON

MODEL

Sphere withpositive chargethroughout

The positive charge of thesphere balances the negativecharge of the electrons.

THOMSONMODEL

RUTHERFORDMODEL

Nucleus

Path of amovingelectron

1900 1905 1910

Negativelycharged particle(electron)

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Students may think that electrons travelaround the nucleus in fixed orbits, likeplanets orbiting the sun. Challenge thismisconception by having studentscompare Bohr’s model and the electroncloud model. Explain that Bohr’s modelcorrectly introduced the concept ofenergy levels, but energy levels cannotbe used to describe the actual locationof an electron. The electron cloud modelcan be used to model the probabilitythat an electron is in a certain location.The exact speed and location of a singleelectron cannot be determined.Verbal

FYIThe usefulness of Bohr’s model waslimited. The model could be used todescribe the behavior of the singleelectron in a hydrogen atom quiteaccurately. However, this model could not be applied to atoms withmultiple electrons.

Integrate Space SciencePlanets in the solar system travel in fixed orbits around the sun. Becausemost of the orbits are nearly circular, the difference between the distance tothe sun when a planet is closest andwhen it is farthest away is not great(given the magnitude of distances inspace). Pluto is an exception. Its orbit isso elliptical that there are times duringPluto’s journey around the sun (249Earth days) when it is closer to the sunthan Neptune is. This switch in order of proximity to the sun lasts 20 years. It last happened between 1979 and1999. Have students research when it will happen again. Logical

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Section 4.3 (continued)

Customize for English Language Learners

Think-Pair-ShareHave students work in pairs to think ofstructures that can serve as analogies forenergy levels. Examples include rungs of aladder, guitar frets, and the series of holes on abelt or shoe strap. Note, however, that in all ofthese models, the intervals are equal, which is

not true of the intervals between energy levels.Provide pictures of dressers that have drawersof different sizes or bookshelves that haveadjustable shelves, which might better modelthe intervals of energy levels. Strengthendiscussion skills by having students share their examples with the class.

The landing at the bottom of the staircase is like the lowest energylevel in an atom. Each step up represents a higher energy level. Thedistance between two steps represents the difference in energy betweentwo energy levels. To continue the analogy, there would need to be adifferent staircase for each element because no two elements have thesame set of energy levels.

An electron in an atom can move from one energy level toanother when the atom gains or loses energy. An electron may moveup two energy levels if it gains the right amount of energy. An elec-tron in a higher energy level may move down two energy levels if itloses the right amount of energy. The size of the jump between energylevels determines the amount of energy gained or lost.

1932 JamesChadwick, a Britishphysicist, confirms theexistence of neutrons,which have no charge.Atomic nuclei containneutrons and positivelycharged protons.

1924 Frenchman Louisde Broglie proposes thatmoving particles likeelectrons have someproperties of waves.Within a few years,evidence is collected tosupport this idea.

1926 Erwin Schrödingerdevelops mathematicalequations to describe themotion of electrons inatoms. His work leads tothe electron cloud model.

ELECTRONCLOUDMODEL

The electron cloud isa visual model of theprobable locations ofelectrons in an atom.The probability offinding an electron ishigher in the denserregions of the cloud.

The nucleuscontains protonsand neutrons.

1915 1920 1925 1930 1935

Electrons gain orlose energywhen they movebetween fixedenergy levels.

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1913 In NielsBohr’s model, theelectrons move inspherical orbits atfixed distancesfrom the nucleus.

BOHRMODEL

Nucleus

Electron

Summary Select a scientistmentioned on the time line.Research and write a para-graph about the scientist’searly years. What experiencesled to his interest in science?Was he the first in his familyto be interested in science?What subjects did he studyat school?

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FYIIn Section 6.1, students will learn thatelectrons sometimes gain enoughenergy to escape from an atom.

Models of the AtomHave groups of students build or drawmodels that represent the changes overtime in scientists’ understanding ofatomic structure. Have them make athree-dimensional version of the timeline shown and display it as a mobile ordiorama. Have students note the timescale on the time line. Explain that thebreak between 1805 and 1895 allowsthe milestone in 1803 to be included.Group, Visual

There will be more information for somescientists than for others. The exercisefocuses on early experiences becausestudents will not understand most ofwhat is written about the careers ofthese scientists. By pooling theirresearch, students will see that scientistscan emerge from diverse backgrounds.

Students may learn that Dalton was the son of a weaver and de Broglie was the son of a duke; that Daltonbegan his teaching career at the age of 12; that Schrödinger was an onlychild, but Rutherford had 11 siblings; or that Chadwick was shy andRutherford charming. Verbal

Use CommunityResourcesHave a female scientist visit the class todiscuss the experiences that led to herinterest in science. Have studentsprepare questions similar to those askedin the Writing in Science feature.Interpersonal, Visual

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Atomic Structure 115

De Broglie and Schrödinger In 1924,Louis de Broglie, a French graduate student,derived an equation that describes thewavelength of a moving particle. Using de Broglie’s equation, an electron has awavelength of about 2 � 10�10 cm. In 1926,Erwin Schrödinger, an Austrian physicist,wrote a mathematical equation to describe the location and energy of the electron in a

hydrogen atom. When the equation is solved(using advanced calculus), it produces a seriesof wave functions that describe the behaviorof electrons. The quantum mechanical modelof atoms is based on these wave functions.Atomic physicists define an orbital as thespace-dependent part of the Schrödinger wavefunction of an electron in an atom or molecule.

Facts and Figures

Evidence for Energy Levels What evidence is there that elec-trons can move from one energy level to another? Scientists canmeasure the energy gained when electrons absorb energy and move toa higher energy level. They can measure the energy released when theelectron returns to a lower energy level.

The movement of electrons between energy levels explains the lightyou see when fireworks explode. Light is a form of energy. Heat pro-duced by the explosion causes some electrons to move to higher energylevels. When those electrons move back to lower energy levels, theyemit energy. Some of that energy is released as visible light. Becauseno two elements have the same set of energy levels, different elementsemit different colors of light.

Electron Cloud ModelLike earlier models, Bohr’s model was improved as scientists madefurther discoveries. Bohr was correct in assigning energy levels to elec-trons. But he was incorrect in assuming that electrons moved likeplanets in a solar system. Today, scientists know that electrons move ina less predictable way.

Scientists must deal with probability when trying to predict thelocations and motions of electrons in atoms. An electron cloud is avisual model of the most likely locations for electrons in an atom. Thecloud is denser at those locations where the probability of finding anelectron is high. Scientists use the electron cloud model todescribe the possible locations of electrons around the nucleus.

Figure 14 provides an analogy for an electron cloud. When the pro-peller of an airplane is at rest, you can count the number of blades.When the propeller is moving, the blades spin so fast that you see onlya blur. You know that the blades are located somewhere in the blur,but at any specific moment in time you can’t be exactly sure whereeach blade is located.

What determines the amount of energy gained orlost when an electron moves between energy levels?

Figure 14 When the propeller ofan airplane is at rest, you can seethe locations of the blades. Whenthe propeller is moving, you seeonly a blur that is similar to adrawing of an electron cloud.Comparing and ContrastingDescribe one difference betweenthe motion of a propeller and themotion of an electron.

116 Chapter 4

For: Links on energy levels

Visit: www.SciLinks.org

Web Code: ccn-1043

Electron Cloud ModelBuild Science SkillsUsing Models Have students examinethe propeller in Figure 14. Ask, How isthe moving propeller similar to anelectron cloud? (You cannot be sure atany specific moment where the propellerblades or electrons are located. However,the central part of the propeller and thenucleus of an atom are in fixed locations.)What other examples can you think ofthat could model the concept of anelectron cloud? (Acceptable answersinclude a ceiling fan, or moths flyingaround a light bulb.) Visual

Electron Cloud Model

Purpose Students will use a model to describe the probable position ofelectrons.

Materials small, round balloon; large, round balloon; 10 beads with 4-mm diameter; 5 beads with 2-mm diameter

Procedure Put the 4-mm beads intothe small balloon. Tell students that thesmall balloon represents the nucleus of aboron atom (five neutrons, five protons).Put the 2-mm beads into the largeballoon. Explain that the beads representelectrons and the balloon represents theelectron cloud. Slightly inflate the smallballoon and push it completely into thelarge balloon. Inflate the large balloonand tie the end. Agitate the balloon sothat the small beads are in constantmotion.

Expected Outcome The preciselocation of a bead at a specific time isunknown, but the probability that it is in the large balloon is quite high. Kinesthetic

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Section 4.3 (continued)

Emission and Absorption Spectra Whenthe energy gained or lost by an atom is lightenergy, each frequency (or wavelength) oflight corresponds to a movement of anelectron between two energy levels in theatom. An element can be identified by thefrequencies of light that are absorbed or

emitted by its atoms because no two elementshave the same set of energy levels. Forexample, the element helium was discoveredon the sun in 1868 before it was discovered onEarth. The spectrum of light emitted by gaseson the surface of the sun contained a yellowline that did not match a known element.

Facts and Figures

116 Chapter 4

Download a worksheet on energylevels for students to complete,and find additional teacher supportfrom NSTA SciLinks.

Atomic Structure 117

Atomic Orbitals The electron cloud represents all the orbitals in an atom. An orbital isa region of space around the nucleus where an electron is likely to befound. To understand the concept of an orbital, imagine a map of yourschool. Suppose you mark your exact location with a dot once every10 minutes over a period of one week. The places you visit the most—such as your classrooms, the cafeteria, and the area near yourlocker—would have the highest concentration of dots. The places youvisit the least would have the lowest concentration of dots.

The dots on your map are a model of your “orbital.” They describeyour most likely locations. There are some locations in your orbitalthat you may not visit every week—such as the principal’s office or theauditorium. These locations may not be represented by a dot on yourmap. Despite such omissions, the dots on your map are a good modelof how you usually behave in your orbital. An electroncloud is a good approximation of how electrons behave intheir orbitals.

The level in which an electron has the least energy—thelowest energy level—has only one orbital. Higher energylevels have more than one orbital. Figure 15 shows thenumber of orbitals in the first four energy levels of an atom.Notice that the maximum number of electrons in an energylevel is twice the number of orbitals. Each orbital can containtwo electrons at most.

Comparing Excited States

Materialsfluorescent (“neon”) markers, glow-in-the-darktoy, ultraviolet (UV) lamp

Procedure1. Use the fluorescent markers to draw a picture

on a piece of paper.

2. With the room darkened, observe yourdrawing under a UV lamp. CAUTION Do notlook directly at the light. Remove the drawingfrom under the UV light and observe it again.Record your observations.

3. Observe the glow-in-the-dark toy under theUV light. Remove the toy from the light andobserve it again. Record your observations.

Analyze and Conclude1. Observing How did the glow of the toy

differ from the glow of your drawing?

2. Formulating Hypotheses Use the conceptsof ground and excited states to explain howUV light caused your drawing and the toyto glow.

3. Drawing Conclusions In which object, yourdrawing or the toy, do the atoms have excitedstates that are more stable, or less likely tochange? Explain your answer.

Figure 15 The table lists thenumber of orbitals in the first fourenergy levels of an atom. It alsolists the maximum number ofelectrons in each energy level.Inferring How many electronscan be in each orbital?

EnergyLevel

Number ofOrbitals

Maximum Numberof Electrons

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Energy Levels, Orbitals,and Electrons

Atomic Orbitals

Comparing ExcitedStates

ObjectiveAfter completing this activity, studentswill be able to • explain how UV light causes objects

to glow.• use the persistence of light to

compare excited states.

Skills Focus Observing, FormulatingHypotheses

Prep Time 5 minutes

Class Time 15 minutes

Safety Check the MSDS for themarkers to make sure that they are lowin VOCs (volatile organic compounds).Students should not look directly at UVlight, which is harmful to the eyes.Demonstrate safe use of the UV lampsbefore allowing students to use them.

Teaching Tips• Do this lab only after you teach

ground state and excited states.

Expected Outcome Fluorescent inkwill emit brilliant visible light under UVlight. This fluorescence will instantlycease when the UV light is removed.Phosphorescent (glow-in-the-dark)objects will continue to emit visible lighteven after the UV light is removed.

Analyze and Conclude 1. The toy still glowed after the UV lightwas removed. The drawing did not. 2. The drawing and the toy absorbedenergy from the UV light. When electronsmoved to higher energy levels, the atomswere in an excited state. When electronsreturned to lower energy levels, energywas released as visible light. 3. The fact that the toy’s glow persistedsuggests that the excited state of atomsin the toy was more stable than theexcited state of atoms in the drawing. Visual, Kinesthetic

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Answer to . . .

Figure 14 The propeller blades havea single, set path, and the blades stopmoving when the engine is shut off.

Figure 15 Two

The size of the jumpbetween energy levels

FYIAccording to the quantum mechanical model, an orbital is the mathematical function thatdescribes the behavior of an electron in space.

118 Chapter 4

Section 4.3 Assessment

Reviewing Concepts1. When is an electron in an atom likely to

move from one energy level to another?

2. What model do scientists use to describehow electrons move around the nucleus?

3. Describe the most stable configurationof the electrons in an atom.

4. What did Bohr contribute to modernatomic theory?

5. What does an electron cloud represent?

Critical Thinking6. Comparing and Contrasting A boron

atom has two electrons in the first energy leveland three in the second energy level. Comparethe relative energies of the electrons in thesetwo energy levels.

7. Making Judgments Was Rutherford’s modelof an atom incorrect or incomplete? Explainyour answer.

8. Posing Questions Apply what you knowabout charged particles to the modern modelof the atom. Is there anything about thebehavior of electrons in atoms that isunexpected? Explain your answer.

Electron ConfigurationsHow are the seats in your classroom arranged? Are they lined up neatlyin rows, or are they grouped in clusters? A configuration is an arrange-ment of objects in a given space. Some configurations are more stablethan others, meaning that they are less likely to change. The positionof the gymnast on the balance beam in Figure 16 is not very stablebecause the beam is only 10 centimeters wide.

An electron configuration is the arrangement of electrons in theorbitals of an atom. The most stable electron configuration is theone in which the electrons are in orbitals with the lowest possibleenergies. When all the electrons in an atom have the lowest possible

energies, the atom is said to be in its ground state.For example, lithium is a silvery-white metal with an atomic

number of 3, which means that a lithium atom has three electrons. Inthe ground state, two of the lithium electrons are in the orbital of thefirst energy level. The third electron is in an orbital of the secondenergy level.

If a lithium atom absorbs enough energy, one of its electrons canmove to an orbital with a higher energy. This configuration is referredto as an excited state. An excited state is less stable than the groundstate. Eventually, the electron that was promoted to a higher energylevel loses energy, and the atom returns to the ground state. Helium,neon, argon, krypton, and xenon atoms returning from excited statesto the ground state emit the light you see in “neon” lights.

Figure 16 A gymnast on abalance beam is like an atom inan excited state—not very stable.

Describing Energy Levels Use a bookcase asan analogy for the energy levels in an atom.Use the analogy to write a paragraph aboutelectrons and energy levels. (Hint: Reread thestaircase analogy on pages 114 and 115.)

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ElectronConfigurationsUse VisualsFigure 16 Extend the analogy of thegymnast on the balance beam by havingstudents consider a gymnast doing anentire routine on equipment such as abalance beam, a pommel horse, parallelbars, or uneven bars. In the analogy,when is the configuration of thegymnast like an atom in an excitedstate? (When the gymnast is in aprecarious position, such as when thegymnast is not in direct contact withthe equipment) In the analogy, whenis the gymnast most like an atomin its ground state? (The gymnast isin her most stable position when sheis standing on the floor.)Logical

ASSESSEvaluateUnderstandingHave students draw and label a diagramthat represents Bohr’s model of anatom. Then, have students explain howthe electron cloud model differs fromBohr’s model.

ReteachUse the diagrams on p. 115 in theScience and History feature to reviewBohr’s model, energy levels, andelectron clouds.

The shelves in a bookcase can representenergy levels in an atom. If studentsknow about potential energy, they maycompare what happens to the energy ofa book as it is moved between shelvesto the difference in energy betweenelectrons in different energy levels.

If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 4.3.

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Section 4.3 (continued)

5. An electron cloud represents the mostprobable locations of an electron in an atom.6. The electrons in the second energy level willhave more energy than the electrons in thefirst energy level.7. Rutherford’s description of an atom wascorrect, but incomplete. It did not provide asmuch information about the behavior of theelectrons as later models.8. Students may ask why the negativelycharged electrons are not drawn into thenucleus by the positively charged protons.

Section 4.3 Assessment

1. Electrons are likely to move from oneenergy level to another when atoms gain orlose energy.2. The electron cloud model3. The most stable configuration is the one inwhich the electrons are in orbitals with thelowest possible energy.4. Bohr contributed the idea that electronshave energy levels with specific amountsof energy.