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100 Chapter 4
4.1 Studying Atoms
Key ConceptsWhat was Dalton’s theoryof the structure of matter?
What contributions didThomson and Rutherfordmake to the developmentof atomic theory?
Vocabulary◆ nucleus
Reading StrategySummarizing Copy thetable. As you read,complete the table aboutatomic models.
Studying the structure of atoms is a little like studying wind. Becauseyou cannot see air, you must use indirect evidence to tell the directionof the wind. You might notice which way fallen leaves move as they arepushed by the wind, and infer that the leaves and wind are moving inthe same direction.
Atoms pose a similar problem because they are extremely small.Even with a microscope, scientists cannot see the structure of an atom.In this chapter, you will find out how John Dalton, J. J. Thomson,Ernest Rutherford, Niels Bohr, and other scientists used evidence fromexperiments to develop models of atoms.
Ancient Greek Models of Atoms If you cut a piece of aluminum foil in half, you have two smaller piecesof the same shiny, flexible substance. You could cut the pieces again andagain. Can you keep dividing the aluminum into smaller pieces? Greekphilosophers debated a similar question about 2500 years ago.
The philosopher Democritus believed that all matter consisted ofextremely small particles that could not be divided. He called theseparticles atoms from the Greek word atomos, which means “uncut” or“indivisible.” He thought there were different types of atoms with spe-cific sets of properties. The atoms in liquids, for example, were roundand smooth, but the atoms in solids were rough and prickly.
Aristotle did not think there was a limit to the number of timesmatter could be divided. Figure 1 shows the model Aristotle used todescribe matter. For many centuries, most people accepted Aristotle’sviews on the structure of matter. But by the 1800s, scientists hadenough data from experiments to support an atomic model of matter.
Rutherford
Ratio of massesin compounds
Positive, densenucleus
Scientist Evidence Model
b. ?a. ?
Deflected beam
e. ?
d. ?c. ?
Fire
Water
DryHot
ColdW
et
Air Earth
Figure 1 Aristotle thought thatall substances were built up fromonly four elements—earth, air,fire, and water. These elementswere a combination of fourqualities—hot, cold, dry, and wet.Fire was a combination of hot anddry. Water was a combination ofcold and wet.
100 Chapter 4
FOCUS
Objectives4.1.1 Describe ancient Greek models
of matter.4.1.2 List the main points of Dalton’s
atomic theory and describe hisevidence for the existence ofatoms.
4.1.3 Explain how Thomson andRutherford used data fromexperiments to produce theiratomic models.
Build VocabularyLatin Plural Forms Explain that theword nucleus comes from a Latin wordmeaning “kernel.” Explain that a kernelis a grain or seed. Ask students to discusshow the definition of the term nucleusrelates to its Latin origin. (Like the kernelof a nut, the nucleus is a small, massivecenter of the atom.) Remind students thatthe plural of the word nucleus is nuclei.
Reading Strategya. Dalton b. Indivisible, solid spheresc. Thomson d. Negative charges evenlyscattered through a positively chargedmass of matter (plum pudding model)e. Deflection of alpha particles passingthrough gold foil
INSTRUCT
Ancient Greek Models of AtomsUse VisualsFigure 1 Have students examine thediagram in Figure 1 that lists thequalities of each of Aristotle’s fourelements. Ask, What qualities didAristotle use to describe air? (Air is acombination of hot and wet.) Whatelement was a combination of dryand cold? (Earth) Is “wet and cold” an accurate description of water?(Wet describes water, but water isn’talways cold.)Visual
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Reading Focus
1
Section 4.1
Print• Reading and Study Workbook With
Math Support, Section 4.1 • Transparencies, Chapter Pretest and
Section 4.1
Technology• Interactive Textbook, Section 4.1• Presentation Pro CD-ROM, Chapter Pretest
and Section 4.1• Go Online, NSTA SciLinks, Atomic theory
Section Resources
Dalton’s Atomic TheoryJohn Dalton was born in England in 1766. He was a teacher who spenthis spare time doing scientific experiments. Because of his interest inpredicting the weather, Dalton studied the behavior of gases in air.Based on the way gases exert pressure, Dalton correctly concluded thata gas consists of individual particles.
Evidence for Atoms Dalton gathered evidence for the existenceof atoms by measuring the masses of elements that combine whencompounds form. He noticed that all compounds have something incommon. No matter how large or small the sample, the ratio of themasses of the elements in the compound is always the same. In otherwords, compounds have a fixed composition.
For example, when magnesium burns, as shown in Figure 2, itcombines with oxygen. The product of this change is a white solidcalled magnesium oxide. A 100-gram sample of magnesium combineswith 65.8 grams of oxygen. A 10-gram sample of magnesium com-bines with 6.58 grams of oxygen. The ratio of the mass of magnesiumto the mass of oxygen is constant in magnesium oxide.
Dalton’s Theory Dalton developed a theory to explain why theelements in a compound always join in the same way. Dalton pro-posed the theory that all matter is made up of individual particlescalled atoms, which cannot be divided. The main points of Dalton’stheory are as follows.
■ All elements are composed of atoms.
■ All atoms of the same element have the same mass, and atoms ofdifferent elements have different masses.
■ Compounds contain atoms of more than one element.
■ In a particular compound, atoms of different elements alwayscombine in the same way.
In the model of atoms based on Dalton’s theory, the elementsare pictured as solid spheres like those in Figure 3. Each type ofatom is represented by a tiny, solid sphere with a different mass.
Recall that a theory must explain the data from many exper-iments. Because Dalton’s atomic theory met that goal, the theorybecame widely accepted. Over time, scientists found that not allof Dalton’s ideas about atoms were completely correct. But this didnot cause later scientists to discard the atomic theory. Instead, theyrevised the theory to take into account new discoveries.
What did Dalton notice that all compounds havein common?
Figure 2 Magnesium reacts withoxygen to form the compoundmagnesium oxide. The ratio ofmagnesium to oxygen, by mass, inmagnesium oxide is always about3 : 2. Observing What color ismagnesium oxide?
Atomic Structure 101
Figure 3 Dalton made thesewooden spheres to represent theatoms of different elements.
Dalton’s AtomicTheoryBuild Science SkillsEvaluating As students read Chapter 4,have them evaluate what portions ofDalton’s model were accurate and whatportions needed to be revised. (Daltondid not discuss subatomic particles, which are smaller components of atoms.Atoms of the same element can havedifferent masses.)Logical
Many students have trouble differen-tiating compounds from mixtures.Remind students that Dalton noticedthat the ratio of masses of elements in a compound is always the same.Compounds are distinguished frommixtures and solutions by their fixedcompositions. Have students recallexamples of mixtures and describe how their compositions can vary. Verbal
Use VisualsFigure 3 Have students examine thewooden spheres shown in Figure 3. Ask, Why do you think there are holes in Dalton’s wooden spheres?(One acceptable answer is that Daltonused the holes in the spheres to connectspheres together to construct models of compounds.)Visual, Logical
FYINot every scientist was convinced thatDalton had the physical evidence tosupport the assumption that elements arecomposed of indivisible particles calledatoms. William Whewell (1784–1868)argued that particles could combine infixed proportions in compounds, but stillnot be indivisible.
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Atomic Structure 101
Customize for English Language Learners
Using Visual Aids Have students draw simple illustrations for the models of atoms according to Dalton,Thompson, and Rutherford. Have them
describe each model to a partner using theillustrations as a visual aid. Then, have eachpair of students discuss the similarities anddifferences among the three models. Answer to . . .
Figure 2 White
Dalton noticed that the ratio of masses of
elements in a compound is always the same.
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102 Chapter 4
Thomson’s Model of the AtomWhen some materials are rubbed, they gain the ability to attract orrepel other materials. Glass and the amber in Figure 4 have this prop-erty. Based on their behavior, such materials are said to have either apositive or a negative electric charge. Objects with like charges repel,or push apart. Objects with opposite charges attract, or pull together.
Some charged particles can flow from one location to another.A flowof charged particles is called an electric current. When you turn on anappliance such as a hair dryer, a current flows from the wall socketthrough the appliance. Joseph John Thomson (1856–1940), better knownas J. J. Thomson, used an electric current to learn more about atoms.
Thomson’s Experiments Thomson used a device like the oneshown in Figure 5A. At the center of the device is a sealed glass tubefrom which most of the air has been removed. There is a metal disk ateach end of the tube. Wires connect the metal disks to a source of elec-tric current. When the current is turned on, one disk becomes negativelycharged and the other disk becomes positively charged. A glowingbeam appears in the space between the disks.
Thomson hypothesized that the beam was a stream of charged par-ticles that interacted with the air in the tube and caused the air to glow.In one experiment Thomson did to test his hypothesis, he placed a pairof charged metal plates on either side of the glass tube, as shown inFigure 5B. The plates caused the beam to deflect, or bend, from itsstraight path. Thomson observed that the beam was repelled by thenegatively charged plate and attracted by the positively charged plate.
Investigating Charged Objects
Materialstransparent tape, metric ruler, scissors
Procedure1. Cut two 10-cm pieces of tape. Fold over 1 cm
of tape at one end of each piece of tape toform a “handle.”
2. Hold the pieces of tape by their folded endsso that they are hanging straight down.Then, without letting the pieces of tape touch,slowly bring their sticky sides close together.Record your observations.
3. Place one piece of tape on a clean surface withthe sticky side facing down.
4. Place the second piece, sticky side down,directly over the first piece, as shown. Pressdown firmly so the pieces stick together.
5. Remove the joined strips from the table.Slowly peel the strips apart.
6. Bring the separated strips close togetherwithout touching. Record your observations.
Analyze and Conclude1. Drawing Conclusions What can you
conclude about the charges on the twopieces of tape after they are separated?
2. Inferring What other objects have youobserved that became charged?
Sticky sides down
Figure 4 Amber is the hardenedform of a sticky, viscous liquidthat protects trees from insectsand disease. If amber is rubbedwith wool, it becomes chargedand can attract a feather.Predicting What will happento the feather if the amber losesits charge?
102 Chapter 4
Thomson’s Model of the Atom
Investigating Charged Objects
Objective After completing this activity, studentswill be able to• explain that like charges repel and
unlike charges attract.
Skills Focus Observing, DrawingConclusions, FormulatingHypotheses
Prep Time 5 minutes
Class Time 10 minutes
Expected Outcome Students willdiscover that the two pieces of tape areattracted to one another after they arepulled apart and then brought near oneanother.
Analyze and Conclude 1. The two pieces of tape have oppositecharges because they attract whenbrought close together. 2. Possible answers include clothes thatcling together when removed from adryer, and the charge that builds upwhen a person walks across a carpet(which is demonstrated by the sparkthat occurs when the person touches a doorknob). Kinesthetic, Logical
FYIIf you do not want to present all theexperimental evidence for the atomictheory, be sure students understandattraction and repulsion of chargedparticles and Rutherford’s nucleus modelof the atom.
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Section 4.1 (continued)
Charge People have known for thousands ofyears that amber can attract other materials after it has been rubbed with fur. Plato evenrefers to these attractive powers of amber in one of his dialogues.
The labels positive and negative werearbitrarily assigned by Benjamin Franklinduring his studies of electric charge andelectric current. (He also was the first to usethe terms battery and conductor.)
Facts and Figures
Evidence for Subatomic Particles Thomson concludedthat the particles in the beam had a negative charge because they wereattracted to the positive plate. He hypothesized that the particles camefrom inside atoms. He had two pieces of evidence to support hishypothesis. No matter what metal Thomson used for the disk, the par-ticles produced were identical. The particles had about the massof a hydrogen atom, the lightest atom.
Thomson’s discovery changed how scientists thought about atoms.Before his experiments, the accepted model of the atom was a solid ballof matter that could not be divided into smaller parts. Thomson’sexperiments provided the first evidence that atoms are made of evensmaller particles. Thomson revised Dalton’s model to account for thesesubatomic particles.
Thomson’s Model An atom is neutral, meaning it has neither anegative nor a positive charge. How can an atom contain negative par-ticles and still be neutral? There must be some positive charge in theatom. In Thomson’s model of the atom, the negative charges wereevenly scattered throughout an atom filled with a positively chargedmass of matter. The model is called the “plum pudding” model, aftera traditional English dessert.
You might prefer to think of Thomson’s model as the “chocolatechip ice cream” model. Think of the chocolate chips in Figure 6 as thenegative particles and the vanilla ice cream as the positively chargedmass of matter. When the chocolate chips are spread evenly through-out the ice cream, their “negative charges” balance out the “positivecharge” of the vanilla ice cream.
How do objects with the same charge behavewhen they come close to one another?
12000
Sealed tube filled withgas at low pressure
Source ofelectric current
Metal disk
–
+
Metal disk
Glowingbeam
A
Figure 5 Thomson used a sealed tube of gas in his experiments.A When the current was on, the disks became charged and aglowing beam appeared in the tube. B The beam bent toward apositively charged plate placed outside the tube. Inferring What was the charge on the particles in the beam?
Figure 6 A scoop of chocolatechip ice cream can representThomson’s model of the atom.The chips represent negativelycharged particles, which arespread evenly through a mass ofpositively charged matter—thevanilla ice cream.
Atomic Structure 103
Positiveplate
Negativeplate
B
Source ofelectric current
Metal disk
–
+
FYIThomson used the speed of an electron,its angle of deflection, and the strengthof the current to determine the charge-to-mass ratio of an electron. RobertMilliken determined the actual mass of an electron through his oil-dropexperiment.
The current that flows through anappliance is an alternating current. Thecurrent Thomson used in his experimentwas a direct current. Not all movementof charge is a current. Charge can flowto or from a balloon (or between a handand a doorknob). With a current, chargemust flow continuously (at least until the circuit is interrupted). The conceptsof electric charge and current areaddressed in depth in Chapter 20.
Use Community ResourcesHave a physics or chemistry professor visitthe class and demonstrate Thomson’sexperiment using a cathode ray tube.Encourage students to think of questionsto ask about how the experimentdemonstrates the properties of electrons. Interpersonal, Visual
Build Reading LiteracyCompare and Contrast Refer topage 226D in Chapter 8, whichprovides the guidelines for comparingand contrasting.
Have students read about the differentatomic models described in Section 4.1.Then, have students create a chart thatcompares and contrasts each model.Logical
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Atomic Structure 103
Answer to . . .
Figure 4 The feather will no longerbe attracted to the amber and willdrop to the ground.
Figure 5 Negative
Objects with the samecharge repel.
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Expected results
Gold atoms
Actual results
Nucleus
Alphaparticles
Alphaparticles
Rutherford’s Atomic TheoryWhen you try something new, you may have expectations about theoutcome. Does the outcome always meet your expectations or are yousometimes surprised? Scientists can also be surprised by the results oftheir experiments, but unexpected results can lead to important dis-coveries. This is what happened to Ernest Rutherford (1871–1937).
Rutherford’s Hypothesis In 1899, Ernest Rutherford discov-ered that uranium emits fast-moving particles that have a positivecharge. He named them alpha particles. In 1909, Rutherford asked oneof his students, Ernest Marsden, to find out what happens to alphaparticles when they pass through a thin sheet of gold.
Recall that in Thomson’s model of the atom, the mass and positivecharge are evenly spread throughout an atom. Based on this model,Rutherford hypothesized that the mass and charge at any location inthe gold would be too small to change the path of an alpha particle. Hepredicted that most particles would travel in a straight path from theirsource to a screen that lit up when struck. Those few that did not passstraight through would be deflected only slightly.
The Gold Foil Experiment Marsden used the equipmentshown in Figure 7. He aimed a narrow beam of alpha particles at thegold. The screen around the gold was made of a material that produceda flash of light when struck by a fast-moving alpha particle. By observ-ing the flash, Marsden could figure out the path of an alpha particleafter it passed through the gold.
Some of the locations of the flashes on the screen did not supportRutherford’s prediction. More particles were deflected than he expected.About one out of every 20,000 was deflected by more than 90 degrees.Some of the alpha particles behaved as though they had struck an objectand bounced straight back.
Figure 7 The path of an alphaparticle can be detected by thelocation of a flash on a screen.Rutherford expected the paths ofthe positively charged alphaparticles that were aimed at thethin gold foil to be affected onlyslightly by the gold atoms. Butmore particles were deflectedthan expected and some particlesbounced straight back.
For: Links on atomic theory
Visit: www.SciLinks.org
Web Code: ccn-1041
104 Chapter 4
Screen
Undeflectedparticle
Gold foil
90�
Source ofalpha particles
Slit
Beam ofalpha
particles
Deflectedparticle
104 Chapter 4
Rutherford’s Atomic TheoryFYIBased on Figure 7, students mightconclude that Marsden used a circularscreen. The screen in the diagramrepresents multiple positions of a smallerscreen that was moved from one positionto another while data was collected.Alpha particles will be identified ashelium nuclei in Chapter 10.
Comparing Atomic Models
Purpose Students will comparedifferent atomic models.
Materials 3 clear, round bowls;flavored gelatin mix; canned blueberries;maraschino cherries
Procedure The day before, prepare theflavored gelatin mix. Divide the liquidgelatin evenly into the three bowls. Chillslightly. Drain the blueberries and addthem to one of the bowls so that theyare distributed as evenly as possible inthe nearly gelled mix. Return the bowlsto the refrigerator. Right before class,place a cherry in the center of the thirdbowl. Have students discuss whichatomic models are represented by eachbowl of gelatin.
Expected Outcome The gelatin alonerepresents Dalton’s atomic model. Thegelatin with blueberries representsThomson’s “plum-pudding” atomicmodel, with the berries representingelectrons. The gelatin with the cherryrepresents Rutherford’s atomic model.Visual, Logical
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Section 4.1 (continued)
In His Own Words In a lecture Rutherfordgave at Cambridge in 1936, he recalled hisreaction when Geiger told him about alphaparticles being scattered backward by the
gold foil. Rutherford described his reaction. “It was almost as incredible as if you fired a15-inch shell at a piece of tissue paper and itcame back and hit you.”
Facts and Figures
Download a worksheet on atomictheory for students to complete,and find additional teacher supportfrom NSTA SciLinks.
Section 4.1 Assessment
Reviewing Concepts1. What theory did Dalton propose about
the structure of matter?
2. What evidence did J. J. Thomson provideabout the structure of an atom?
3. What did Rutherford discover about thestructure of an atom?
4. What evidence did Thomson have that hisglowing beam contained negative particles?
5. Why was Dalton’s model of the atom changedafter Thomson’s experiment?
Critical Thinking6. Comparing and Contrasting Explain why
scientists accepted Dalton’s atomic theory butnot the idea of an atom proposed by theGreek philosophers.
7. Drawing Conclusions If you observeda beam of particles being bent toward anegatively charged plate, what mightyou conclude?
8. Relating Cause and Effect In theRutherford experiment, why weren’t allthe alpha particles deflected?
Discovery of the Nucleus The alpha particles whosepaths were deflected must have come close to another chargedobject. The closer they came, the greater the deflection was.But many alpha particles passed through the gold withoutbeing deflected. From these results, Rutherford con-cluded that the positive charge of an atom is not evenlyspread throughout the atom. It is concentrated in avery small, central area that Rutherford called thenucleus. The nucleus is a dense, positively chargedmass located in the center of the atom. (The plural ofnucleus is nuclei.)
Because Thomson’s model no longer explainedall the evidence, Rutherford proposed a new model.
According to Rutherford’s model, all of an atom’spositive charge is concentrated in its nucleus. The alphaparticles whose paths were deflected by more than 90 degreescame very close to a nucleus. The alpha particles whose paths werenot bent moved through the space surrounding the nuclei withoutcoming very close to any nucleus.
Figure 8 shows the inside of the Astrodome, a domed stadium inHouston, Texas. The roof of the stadium rises to a height of 202 feetabove the center of the field. If an atom had the same volume as the sta-dium, its nucleus would have the volume of a marble. The total volumeof an atom is about a trillion (1012) times the volume of its nucleus.
Atomic Structure 105
Writing to Persuade Imagine you live inancient Greece. Assume all you know aboutmatter is what you can observe with your fivesenses. You have heard the views of bothDemocritus and Aristotle about matter. Writea paragraph supporting one of their views.
Figure 8 The Houston Astrodomeoccupies more than nine acres andseats 60,000 people. If the stadiumwere a model for an atom, amarble could represent its nucleus. Using Analogies In the model,where would the marble have tobe located in the stadium torepresent the nucleus?
Build Science SkillsUsing Models Have students model thegold foil experiment by shooting marblesacross the floor at an arrangement ofwidely spaced, small objects—such asbeads that are glued to a flat surface—and recording the angle of the marblesthat are deflected. Discuss the results oftheir experiment in light of Marsden’sfindings. Kinesthetic, Interpersonal
Integrate BiologyCells in most living organisms have acentral structure called a nucleus. Thisorganelle contains the cell’s genetic, orhereditary, material. Ask, How are anatom’s nucleus and a cell’s nucleussimilar? (They are both central structuresof a basic unit.)Logical
ASSESSEvaluate UnderstandingAsk groups of students to summarizeDalton’s, Thompson’s, and Rutherford’satomic theories. Have them come upwith simple word phrases or mnemonicdevices to help them easily distinguishamong the three theories. For example,Dogs Sort Socks � Dalton’s Solid Sphere;Turtles Play Ping-pong � Thomson’sPlum Pudding; and Rats Poke Noodles �Rutherford’s Positive Nucleus.
ReteachUse the Science and History time line on p. 114 to present and discuss asummary of the three models.
Students might argue that theproperties Democritus assigned toatoms match observed properties ofmatter, such as smoothness androughness. Students might argue thatthe properties Aristotle assigned toelements serve a similar purpose, and his system also seems to account forchanges between types of matter.
If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 4.1.
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Atomic Structure 105
5. Dalton assumed atoms were solid,indivisible particles. Thomson had evidencethat smaller particles existed inside atoms. 6. Dalton had data from experiments tosupport his theory, whereas the Greeks did not have data.7. The particles have a positive charge.8. The nucleus is small compared with theatom as a whole. Very few of the alphaparticles came close enough to a gold nucleusto be deflected.
Section 4.1 Assessment
1. All matter is composed of individualparticles called atoms, which cannot bedivided.2. Thomson provided the first evidence thatatoms are made from even smaller particles.3. All of the positive charge of an atom isconcentrated in its nucleus. 4. The beam was attracted to a positivelycharged plate and repelled by a negativelycharged plate.
Answer to . . .
Figure 8 The marble would have tobe located in the center of the stadium.
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Small-Scale ConstructionSome scientists and engineers think of atoms and molecules asconstruction materials for building very small objects.
Building from the bottom upWith a scanning tunnelingmicroscope, it is possible to moveindividual atoms or molecules.This figure, made of linked carbonmonoxide molecules, is just fivenanometers (0.000005 mm) tall. In1990, scientists built this figure todemonstrate bottom-upconstruction methods.
Building from the top downGears are toothed disks that aredesigned to fit together so that themotion of one gear controls themotion of another. These silicon gearsare among the smallest objects evermade from the top down.
The field of science called nanotechnology is named for a unitof measurement—the nanometer. A billion nanometers (109 nm)can fit in one meter. The diameter of a human hair is about80,000 nm. Scientists and engineers who use nanometers asmeasuring units are building miniature versions of objectssuch as motors.
There are two general methods for building any object.You can start with more material than you need andshape the object by removing matter or you canbuild up the object from smaller pieces. When youshape your nails with an emery board, you are using atop-down construction method. When you see “someassembly required” on the side of a box, the manufacturerexpects you to use a bottom-up method of construction.
Silicon gearassembly
Dust mite
106 Chapter 4
106 Chapter 4
Small-Scale ConstructionBackgroundThe diameter of one of the silicon gearsis 50 microns (50,000 nanometers).Despite their relative sizes on the page, the silicon gear is much largerthan the model of nanogears. (Theatomic radius of a carbon atom is only770 nanometers.)
In 1959, Richard Feynman suggestedthe possibility of nanometer-scaleconstruction when he said, “Theprinciples of physics, as far as I can see,do not speak against the possibility ofmaneuvering things atom by atom.”
The uses for small-scale constructionare potentially very diverse, thoughcurrently they are very limited. Biologistsalready use nanotechnology to maketiny labels for use in diagnostic andpharmaceutical research. Suchmolecular labels can be injected orabsorbed in one part of the body andthen traced as they travel throughoutthe body. One company has even madea sunscreen with nanometer-sizedparticles that scatter harmful light rayswhile transmitting visible light, whichmakes the cream clear instead of white.
Build Science Skills
Using Models
Purpose Students will explore the difference between top-down and bottom-up methods of construction.
Materials modeling clay, plastic knives
Class Time 10 minutes
Procedure Have students use the top-down method to construct a model of agear like the one shown in the dust mitephoto. Consider bringing in samples ofgears for students to observe. Then,have students use the bottom-upmethod to construct a model of thestick figure shown in the feature.
Expected Outcome Students willmake a gear by starting with a lump ofclay and then removing clay to shapethe gear (a top-down constructionmethod). They will make a stick figureby joining small pieces of clay together(a bottom-up method of construction).Kinesthetic, Visual
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� Research proposed uses of nanotechnology.Make a poster describing one proposed use.Explain the advantage ofusing small objects in thisapplication. What problemsmust be solved before theapplication can be used?
� Take a Discovery ChannelVideo Field Trip by watching“Go For Gold.”
Going Further
Video Field Trip
NanogearsThis image of nanogears was produced with a computer program designed to make models of molecules. Hollow tubes(nanotubes) made from sheets of carbonatoms do exist. So do the rings containingcarbon and hydrogen atoms, which are usedfor the “teeth” of the gears. But researchersneed to figure out how to get the “teeth”to attach to the tubes.
The future ofnanotechnology
Potential applications fornanotechnology include
medical diagnostic tools and atomic-level electronic devices that
assemble themselves. If such devicesprove successful, perhaps someday
the surgical robot will be built.
Sheet of carbon atomsrolled into a tube
Futuristic model of ananorobot performing
surgery in a blood vessel.
Ring of carbon atoms withhydrogen atoms attached
Atomic Structure 107
Going FurtherPossible uses include bar codes thatattach to molecules and are used to monitor biological processes, devices that deliver drugs to specificlocations in the body at specific times,and self-assembling products. Oneoverall challenge is the construction of interfaces between humans and tiny devices. Verbal, Visual
Atomic Structure 107
After students have viewed the Video Field Trip,ask them the following questions: What did theancient Egyptians know about how gold reactswith air? (They realized that gold does not reacteasily with air and does not corrode like many othermetals.) Name a practical reason why templeroofs were often covered with gold in countrieswith hot climates. (Because gold is an excellent
reflector of sunlight, it kept the interiors cooler thanthey would have been otherwise.) How do theproperties of gold make it useful on spacemissions? (Gold is often used to coat space vehiclesin order to reflect the intense sunlight. A thin layer ofgold on the goggles used by astronauts reduces theintensity of light reaching the astronauts’ eyes.)How can the arrangement of atoms on thesurface of gold be observed? (Electronmicroscopes are designed to trace the arrangementof particles on a surface through the up-and-downmovement of a probe tip.)
Video Field Trip
Go For Gold
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4.2 The Structure of an Atom
Reading StrategyMonitoring Your Understanding Beforeyou read, copy the table. List what you knowabout atoms and what you would like to learn.After you read, list what you have learned.
Key ConceptsWhat are three subatomicparticles?
What properties can beused to compare protons,electrons, and neutrons?
How are atoms of oneelement different fromatoms of other elements?
What is the differencebetween two isotopes ofthe same element?
Vocabulary◆ proton◆ electron◆ neutron◆ atomic number◆ mass number◆ isotopes
Beams like the ones Thomson produced create the images on manytelevision screens. When a beam sweeps across the screen, spots on thescreen light up in the same way the screen in the gold-foil experimentlit up when struck by an alpha particle. In a color television, there arethree beams, one for each primary color of light—red, green, and blue.The particles in these beams are subatomic particles.
Properties of Subatomic ParticlesBy 1920, Rutherford had seen evidence for the existence of two sub-atomic particles and had predicted the existence of a third particle.
Protons, electrons, and neutrons are subatomic particles.
Protons Based on experiments with elements other than gold,Rutherford concluded that the amount of positive charge varies amongelements. Each nucleus must contain at least one particle with a posi-tive charge. Rutherford called these particles protons. A proton is apositively charged subatomic particle that is found in the nucleus of anatom. Each proton is assigned a charge of 1�. Some nuclei containmore than 100 protons.
Electrons The particles that Thomson detected were later namedelectrons. Electron comes from a Greek word meaning “amber.” Anelectron is a negatively charged subatomic particle that is found in thespace outside the nucleus. Each electron has a charge of 1�.
What I Know What I Would What I HaveAbout Atoms Like to Learn Learned
Figure 9 This 45-foot-tall steelsculpture of a clothespin is inPhiladelphia, Pennsylvania. ClaesOldenburg made the clothespinin 1976 from 10 tons of steel. If aproton had a mass of 10 tons,then an electron would have amass of about 5 kilograms.
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FOCUS
Objectives4.2.1 Identify three subatomic
particles and compare theirproperties.
4.2.2 Distinguish the atomic numberof an element from the massnumber of an isotope, and usethese numbers to describe thestructure of atoms.
Build VocabularyWord-Part Analysis Have studentslook up the term isotope in a dictionarythat provides word prefixes. Have themuse the prefix iso- to help themunderstand the term. (The prefix iso-means “same.” Isotopes of an elementhave the same atomic number, butdifferent numbers of neutrons.)
Reading StrategyMost students will know that atoms arethe “building blocks” of matter, andsome may know that atoms containsubatomic particles. Students may saythat they want to learn more about thestructure of atoms.
INSTRUCT
Properties ofSubatomic ParticlesUse VisualsFigure 9 Have students examine thephoto and the caption that describes it in Figure 9. Suggest some familiarobjects that have a mass of 5 kg, such as a 12-pack of 16-ounce beveragecontainers. Ask, If a proton’s mass was10 tons and an electron’s mass was 5 kg, what mass would represent themass of a neutron? (10 tons)Logical
FYIDifferent books use different conventionsfor symbols for subatomic particles. Forexample, some texts use only the letters e, p, and n. Others include a superscriptzero on the n to indicate the lack ofcharge. Electrons, protons, and neutronsare not the only subatomic particles.Quarks will be discussed in Chapter 10.
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Reading Focus
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Section 4.2
Print• Reading and Study Workbook With
Math Support, Section 4.2 and Math Skill: Electrons and Orbitals
• Transparencies, Section 4.2
Technology• Interactive Textbook, Section 4.2• Presentation Pro CD-ROM, Section 4.2• Go Online, Science News, Atomic chemistry
Section Resources
Figure 10 This table lists thesymbol, the relative charge, therelative mass, and the actual massof an electron, a proton, and aneutron. The Model columnshows the colors used in this bookto represent the subatomicparticles. Calculating What isthe difference in actual massbetween a proton and a neutron?
Designing an Atomic Exhibit
You work as a volunteer at the local sciencemuseum. You are asked to design an exhibit thatcompares the size of a lithium atom to the size ofits nucleus. A lithium atom has a diameter of about3 � 102 picometers. The nucleus of a lithium atomhas a diameter of about 5 � 10
_3 picometers.There are a trillion (1012) picometers in a meter.
Defining the Problem State the problem inyour own words. What decisions will you needto make before you can proceed?
Organizing Information How many timeslarger is the lithium atom than its nucleus? Findseveral objects that could represent the nucleusin your exhibit and measure their diameters.
Creating a Solution Pick one of the objects youmeasured to represent the nucleus in your atomicexhibit. Figure out how far away from the objectyou would have to place a marker so that peoplecould visualize the relative sizes of the atom andthe nucleus.
Presenting Your Plan Write a proposal topresent to the committee that approves projects.Tell them where you would place the nucleus andwhere you would have to place the marker. Beprepared to explain why your exhibit needs thespace you are requesting.
Particle Symbol RelativeCharge
Relative Mass(proton � 1)
Actual Mass(g)
Electron
Proton
Neutron
e�
p�
n
1�
1�
0
1
1
9.11 � 10�28
1.674 � 10�24
1.675 � 10�24
11836
Model
Properties of Subatomic Particles
Atomic Structure 109
Neutrons In 1932, the English physicist James Chadwick designedan experiment to show that neutrons exist. Chadwick concluded thatthe particles he produced were neutral because a charged object didnot deflect their paths. A neutron is a neutral subatomic particle thatis found in the nucleus of an atom. It has a mass almost exactly equalto that of a proton.
Comparing Subatomic ParticlesFigure 10 summarizes some properties of protons, electrons, and neu-trons. Protons, electrons, and neutrons can be distinguished bymass, charge, and location in an atom. Protons and neutrons havealmost the same mass. But the data in Figure 10 show that it wouldtake about 2000 electrons to equal the mass of one proton. Electronshave a charge that is equal in size to, but the opposite of, the charge ofa proton. Neutrons have no charge. Protons and neutrons are found inthe nucleus, but electrons are found in the space outside the nucleus.
For: Articles on atomic chemistry
Visit: PHSchool.com
Web Code: cce-1042
Comparing SubatomicParticlesBuild Science SkillsCalculating Have students confirm the relative masses given in Figure 10 bydividing the mass given for an electronby the mass given for a neutron. (9.11� 10�28/1.675 � 10�24 = 5.44 � 10�4; 1/1836 = 5.45 � 10�4)These numbers are almost equal. Notethat the masses given are in grams. Logical
Designing an Atomic Exhibit
Defining the Problem To design an exhibit that compares the size of alithium atom to the size of its nucleus,students must decide what materials touse and where to locate the exhibit.
Organizing Information It is 60,000times larger than its nucleus.
Creating a Solution If a standardmarble with a 5/8-inch diameter (1.6 cm)represents the nucleus, the marker shouldbe at a distance of 960 m to representthe outer limit of the atom.
Presenting Your Plan The proposalshould explain why the exhibit requiresat least two locations that are about akilometer apart to make a model of thelithium atom in which the nucleus is thesize of a marble. Encourage students tolocate two familiar area landmarks thatare the appropriate distance apart basedon the item chosen for the nucleus.Visual, Logical
For Extra HelpBe sure that students multiply by theconversion factor that has the given unitin the denominator and the desired unitin the numerator.Logical
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Atomic Structure 109
Customize for Inclusion Students
Visually ImpairedProvide visually impaired students with atactile model that represents the difference in mass between a proton and an electron.Count out (or have a group of students countout) 1836 small beads and place them into a
resealable plastic bag. Have them feel thedifference in mass between the bag of 1836 beads and an identical bag containingonly one bead. Point out that the masses they sense include the masses of the bags.
Science News provides studentswith current information onatomic chemistry.
Answer to . . .
Figure 10 0.001 �10�24 g
0098_hsps09te_Ch04.qxp 4/18/07 10:47 AM Page 109
Everything scientists know about the nucleus and subatomic par-ticles is based on how the particles behave. Scientists still do not havean instrument that can show the inside of an atom. But they dohave microscopes that can show how atoms are arranged on the sur-face of a material. The How It Works box on page 111 describes one ofthose microscopes.
Atomic Number and Mass NumberDalton predicted that the atoms of any element are different from theatoms of all other elements. With the discovery of subatomic particles,scientists were able to describe those differences.
Atomic Number The atoms of any given element alwayshave the same number of protons. For example, there is oneproton in the nucleus of each and every hydrogen atom.Therefore, hydrogen is assigned the atomic number 1. Theatomic number of an element equals the number of protonsin an atom of that element.
Hydrogen atoms are the only atoms with a single proton.Atoms of different elements have different numbers of
protons. The sulfur shown in Figure 11A is assigned atomicnumber 16 because a sulfur atom has 16 protons. You can useatomic numbers to refer to elements, like names and symbols,because each element has a unique atomic number.
Each positive charge in an atom is balanced by a negativecharge because atoms are neutral. So the atomic number of anelement also equals the number of electrons in an atom. Eachhydrogen atom has one electron. Each sulfur atom has 16.
Mass Number The atomic number tells you the numberof protons in an atom’s nucleus. It does not give you any infor-mation about the number of neutrons in an atom. For thatinformation, you need to know the atom’s mass number. Themass number of an atom is the sum of the protons and neu-trons in the nucleus of that atom. An atom of aluminum with13 protons and 14 neutrons has a mass number of 27. If youknow the atomic number and the mass number of an atom,you can find the number of neutrons by subtracting.
Number of NeutronsNumber of neutrons � Mass number � Atomic number
Which scientist demonstrated the existence of neutrons?
Figure 11 Each element has adifferent atomic number. A Theatomic number of sulfur (S) is 16.B The atomic number of iron (Fe)is 26. C The atomic number ofsilver (Ag) is 47. Applying Concepts How manyprotons are there in each atom ofsulfur, iron, and silver?
110 Chapter 4
A
B
C
110 Chapter 4
Atomic Number andMass NumberFYIThe atomic mass unit will be introducedin Section 5.2, when atomic masseslisted in the periodic table are discussed.The force that binds protons andneutrons together in the nucleus iscalled the strong nuclear force and isaddressed in Chapter 10, as is the effectof the size of a nucleus on its stability.
Particles and Numbers
Purpose Students will observe therelationship between number ofprotons, number of neutrons, atomicnumber, and mass number.
Materials overhead projector, red andgreen gummy candies
Procedure Explain that the greencandies represent neutrons and the redcandies represent protons. Model alithium-7 nucleus by placing a group of three red candies and four greencandies on the overhead. Ask studentsto count the number of candies(particles) to determine the massnumber of the lithium atom. Then,remove (subtract) the green candies(neutrons) to get the atomic number.Perform a similar demonstration withoxygen-16 (eight protons and eightneutrons) and boron-11 (five protonsand six neutrons).
Expected Outcome Students shouldgain a familiarity with determining massnumbers and atomic numbers. Visual
Build Reading LiteracyIdentify Main Idea/Details Refer to page 98D in this chapter, whichprovides the guidelines for identifyingmain ideas and details.
Have students read Atomic Number andMass Number on p. 110. Ask them toidentify the main idea of each paragraph.Point out that the main idea is usuallywithin the first or second sentence of aparagraph. Encourage students toinclude this exercise in the notes they use to study. Verbal
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Section 4.2 (continued)
Scanning TunnelingMicroscopeA probe is moved back and forth across the surface of asample. When electrons jump, or tunnel, across the gapbetween the sample and the probe, an electric current isproduced. A computer uses data about changes in theprobe’s position to produce an image of the sample’ssurface. Interpreting Diagrams How is the distancebetween the probe tip and the sample kept constant?
Scanning probeAs the probe is moved
over the sample, currentflows between the probetip and the sample. Theprocessor holds the tip at aconstant distance from thesample by keeping theelectric current constant.Thus, changes in the verticalposition of the probe willfollow the contours of thesample’s surface.
ProcessorThe processor sends,
receives, and recordsinformation about themovement of the probe.
Electricalsignal fromprobe
Scanning tunnelingmicroscope Modern scanning tunnelingmicroscopes produce images ofmetal samples or biologicalspecimens such as DNA.
Probe tip The tip ofthe probe is only one ortwo atoms in width.
Electricalsignal fromprocessor
Goldsample
Scanningdevice Thisdevice raisesand lowersthe probe.
Computer A computerassembles a map of the
sample’s surface, using datareceived from the processor.Color was added to theimage shown on thecomputer screen.
Electron flowElectrons flow across a gapof about one nanometer(0.000001 mm)between theprobe tip andthe sample,producingan electriccurrent.
Atomic Structure 111
Scanning TunnelingMicroscopeThe scanning tunneling microscope(STM) is used to obtain high-resolutionimages of solid surfaces. This technologyallows scientists and researchers to viewa three-dimensional profile of a surface,which can give information aboutsurface textures and crystal structure.STM data is initially displayed as a blackand white image that is colorized tohighlight different features.
In 1986, Gerd Binnig of Germany andHeinrich Rohrer of Switzerland shared theNobel Prize in Physics with Germany’sErnst Ruska for designing the scanningtunneling microscope.
Interpreting Diagrams The processormaintains a constant electric currentbetween the probe tip and the sample,which keeps the distance between thetip and sample constant. Logical
For EnrichmentEncourage students to explore the useof scanning tunneling microscopes inresearch on surface textures, crystalstructure, or molecular shape. Havethem present their findings to the classin the form of a poster. Visual
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Atomic Structure 111
Coining Terms The English physicianWilliam Gilbert (1544–1603) introduced theterm electric, which is based on the Greekword for amber. (William Gilbert was thepersonal physician to Queen Elizabeth I and
a pioneer in the study of magnetism. A unit ofmagnetic force is named for him.) Credit fornaming the electron goes to G. JohnstoneStoney, an Irish physicist who suggested thename in 1891.
Facts and Figures
Answer to . . .
Figure 11 There are 16 protons in asulfur atom, 26 in an iron atom, and47 in a silver atom.
James Chadwick
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112 Chapter 4
Section 4.2 Assessment
Reviewing Concepts1. Name three subatomic particles.
2. Name three properties you could use todistinguish a proton from an electron.
3. Which characteristic of an atom alwaysvaries among atoms of different elements?
4. How are the isotopes of an elementdifferent from one another?
5. What do neutrons and protons have incommon? How are they different?
6. How can atoms be neutral if they containcharged particles?
7. What is the difference between atoms ofoxygen-16 and oxygen-17?
Critical Thinking8. Comparing and Contrasting What
property do protons and electrons havethat neutrons do not?
9. Applying Concepts Explain why it isn’tpossible for an atom to have a mass numberof 10 and an atomic number of 12.
IsotopesIn Dalton’s atomic theory, all the atoms of a given element areidentical. Every atom of a given element does have the samenumber of protons and electrons. But every atom of a givenelement does not have the same number of neutrons. Isotopesare atoms of the same element that have different numbers ofneutrons and different mass numbers. Isotopes of an ele-ment have the same atomic number but different massnumbers because they have different numbers of neutrons.
For example, every atom of oxygen has 8 protons. Some oxygenatoms have 8 neutrons and a mass number of 16. Some oxygen atomshave 9 neutrons and a mass number of 17. Some oxygen atoms have10 neutrons and a mass number of 18. When it is important to distin-guish one oxygen isotope from another, the isotopes are referred to asoxygen-16, oxygen-17, and oxygen-18. All three oxygen isotopes canreact with hydrogen to form water or combine with iron to form rust.
With most elements, it is hard to notice any differences in the phys-ical or chemical properties of their isotopes. Hydrogen is an exception.Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.)Hydrogen-2 has one neutron, and hydrogen-3 has two neutrons.Because a hydrogen-1 atom has only one proton, adding a neutrondoubles its mass. Water that contains hydrogen-2 atoms in place ofhydrogen-1 atoms is called heavy water. Figure 12 compares somephysical properties of ordinary water and heavy water.
Elements In Section 2.1, you were toldthat elements contain only one type ofatom. How would you define “type ofatom” to account for the existenceof isotopes?
Property
Meltingpoint
Boilingpoint
Density(at 25�C)
Ordinary Water
0.00�C
100.00�C
0.99701 g/cm3
Heavy Water
3.81�C
101.42�C
1.1044 g/cm3
Comparing Ordinary Waterand Heavy Water
Figure 12 Heavy water containshydrogen-2 atoms, which havetwice the mass of hydrogen-1atoms. Using Tables At whattemperature would a sample ofheavy water freeze?
IsotopesBuild Science SkillsCalculating Uranium-238 has a massnumber of 238 with 146 neutrons in thenucleus. Uranium-235 has 143 neutronsin the nucleus. Ask, What is the atomicnumber of uranium? (92)Logical
Many students think that isotopes aredifferent from “ordinary” or “regular”atoms. To challenge this misconception,have students read the text on this pageand examine the data presented inFigure 12. Ask, How are the composi-tions of heavy water and ordinarywater similar? (Both contain hydrogenand oxygen atoms.) What type ofhydrogen atoms does ordinary watercontain? (Hydrogen-1 atoms) What typeof hydrogen atoms does heavy watercontain? (Hydrogen-2 atoms) Comparethe properties of heavy water andordinary water. (They have differentmelting points, boiling points, anddensities.) Logical
ASSESSEvaluate UnderstandingHave students write three reviewquestions for this section. Studentsshould then break into groups of three orfour and ask each other their questions.
ReteachRevisit Figure 10 to review the differencesamong protons, neutrons, and electrons.
Students might say that “type of atom”refers to the atomic number of the atomor to the number of protons in the atom.
If your class subscribes tothe Interactive Textbook, use it toreview key concepts in Section 4.2.
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Section 4.2 (continued)
the atom. Protons are charged particles.Neutrons are neutral particles. 6. The positive charge of the protons in thenucleus is balanced by the negative charge of the electrons. 7. Each oxygen-17 atom has one moreneutron than each oxygen-16 atom. 8. Protons and electrons are charged particles.Neutrons have no charge. 9. An atom with an atomic number of 12 has12 protons. Because the mass number is thesum of the protons and neutrons, the massnumber would need to be at least 12.
Section 4.2 Assessment
1. Proton, electron, and neutron2. Mass, charge, and location in an atom 3. The atoms of any element have a differentnumber of protons than the atoms of all other elements. 4. Isotopes of an element have the sameatomic number but different mass numbersbecause they have different numbers of neutrons. 5. Protons and neutrons have almost the samemass and are both located in the nucleus of
112 Chapter 4
Answer to . . .
Figure 12 3.81°C
4.3 Modern Atomic Theory
Reading Strategy Sequencing 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.
Atomic Structure 113
FOCUS
Objectives4.3.1 Describe Bohr’s model of the
atom and the evidence forenergy levels.
4.3.2 Explain how the electron cloud model represents thebehavior and locations ofelectrons in atoms.
4.3.3 Distinguish the ground state from excited states of an atom based on electron 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 up a 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 of the AtomBuild Reading LiteracyRelate Text and Visuals Refer topage 190D in Chapter 7, whichprovides the guidelines for relating text 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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Reading Focus
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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
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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)
-
-
---
- -
-++ +
+ ++
+ +
1805 18951800
114 Chapter 4
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- -
-
--
--
-
114 Chapter 4
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-Share Have 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.
- --
-
--
-
-
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?
Atomic Structure 115
+
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 Community ResourcesHave 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
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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
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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
Materials fluorescent (“neon”) markers, glow-in-the-darktoy, ultraviolet (UV) lamp
Procedure 1. 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
1
2
3
4
1
4
9
16
2
8
18
32
Energy Levels, Orbitals,and Electrons
Atomic Orbitals
Comparing Excited States
Objective After 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
L2
Atomic Structure 117
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.
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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 Configurations How 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.)
118 Chapter 4
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 with the equipment) In the analogy, when is the gymnast most like an atom in its ground state? (The gymnast is in her most stable position when she is standing on the floor.)Logical
ASSESSEvaluate UnderstandingHave 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 shelves to 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.
L1
L2
3
L1
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 amounts of energy.