Science, Art, and AtomsRecent advances in technology have made it possible to makeimages of individual particles and even to pick up the particles andmove them around. Some artists who are also scientists have usedindividual particles to create the tiniest works of art in existence,such as the Chinese characters shown in Figure 5.2. This artworkis much too small to ever be seen with the unaided eye. About 100 000 copies laid end to end would be needed to cover thedistance across the diameter of a human hair.

Both Figures 5.1 and 5.2 show artwork made from two differentmetals — iron and copper. The Chinese characters in Figure 5.2 aremade from particles of iron, and the background is made of copperparticles. The individual particles of each element, called atoms, are visible as small bumps in the image. An atom is the smallest part of an element that has all of the element’s properties. Creativescientists and artists are finding new ways to put atoms together.

Atoms of copper are not the same as atoms of iron. This iswhy a piece of copper metal has different properties than a pieceof iron metal. Iron’s strength is useful to artists because it can beused to support heavy weights. Copper has an attractive colourand lustre, and its malleability makes it easy to work with.

168 UNIT B Atoms, Elements, and Compounds

Here is a summary of what youwill learn in this section:

• An element is a puresubstance that cannot bebroken down into othersubstances by chemicalreactions. The smallest piece ofany element having all of thatelement’s properties is oneatom.

• Different models of the atomhave evolved over time asexperiments have revealed newinformation.

• Atoms are composed ofsubatomic particles: negativelycharged electrons, positivelycharged protons, and neutrons,which have no electric charge.

Developing the Atomic Theory

Figure 5.1 Copper and iron are both metals and are both made of tiny particles. However, theparticles in copper are different from the particles in iron.

5.1

Figure 5.2 Made of individual ironatoms on a base of copper atoms,this is an enlargement of one of thesmallest pieces of art. Thecharacters mean “atom” inChinese.

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169The periodic table organizes elements by patterns in properties and atomic structure.

B10 Quick Lab

Calcium Metal in Water

Like copper and iron, the element calcium is a metal.However, calcium is easy to tell apart from metals likecopper and iron.

PurposeTo observe how calcium metal reacts with water

Procedure

1. Fill a beaker with about 300 mL of water.Completely fill a test tube with water. Place arubber stopper over the opening of the test tube,then place the test tube upside down in thebeaker. Reach into the water, and remove therubber stopper. Try not to let any air into the testtube.

2. Dry your hands well. Your teacher will give you afew pieces of calcium metal on a paper towel.Use forceps to drop a piece of calcium metal intothe water. Adjust the position of the test tube sothat the mouth of the test tube covers thecalcium metal. Observe what happens.

3. Light a wooden splint.

4. Use clamps or tongs to lift the test tube out of thewater without turning it upright. Place the flamingsplint under the mouth of the test tube, andobserve what happens.

5. Repeat step 1 with a clean beaker. Add five dropsof phenolphthalein to the water in the beaker,and then repeat steps 2 through 4.

6. Clean up your work area. Follow your teacher’sinstructions to safely dispose of all materialsused. Wash your hands thoroughly.

Questions

7. Why is it important to keep the test tube upsidedown after removing it from over the piece ofcalcium metal?

8. The gas produced in this experiment washydrogen gas. Briefly describe the procedure fortesting for hydrogen gas.

9. How does the phenolphthalein indicator solutionrespond when calcium reacts with water?

CAUTION: Keep your hair tied back when working nearopen flames. Do not touch calcium metal with your barehands as the metal will react with moisture in your skin.

• two 400-mL beakers

• 2 medium test tubes

• 2 rubber stoppers

• water

• pieces of calcium metal

• paper towel

• forceps

• candle and matchesor lighter

• wooden splints

• test-tube clamp ortongs

• phenolphthaleinindicator solution

• medicine dropper

Materials & Equipment

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Evolving Models of the AtomDifferent kinds of atoms give elements different properties.Atomic theory is the study of the nature of atoms and how theycombine to form all types of matter. Atomic theory helps us tounderstand why there are different kinds of atoms. It explainshow atoms combine to form over 100 known elements and allother forms of matter, including compounds and mixtures.

The idea that most of the matter we encounter is made fromcombinations of simple forms of matter is very ancient. Thephilosophers of ancient Greece reasoned that the basic forms ofmatter, which they called elements, were fire, water, earth, andair. In ancient China, the elements were thought to be fire, water,wood, metal, and earth (Figure 5.3). Ancient civilizations usedthese and similar ideas as the basis for understanding the worldand practising medicine. Today, we still use the term element,though in a different way. For example, we still believe that mostsubstances are built up from simpler ones.

About 440 B.C.E., the Greek philosopher Democritushypothesized that breaking down rock into powder and thengrinding the powder further would reduce it to tiny bits of matterthat could not be broken down any more. His idea was not popularand, at the time, there was no experimental evidence to support it.

170 UNIT B Atoms, Elements, and Compounds

Figure 5.3 An ancient Chinese idea about matter is that it is formed from five elementsthat interact in particular ways.

WORDS MATTER

We get the modern term “atom” fromthe Greek atomos, meaningindivisible.

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Atomic Theory Takes ShapeAtomic theory is rooted in the idea that an understanding ofatoms and their structure can help us predict many of theproperties of matter. Modern atomic theory began to take shapein the early 1800s. It was then that John Dalton (1766–1844), ascientist and teacher in England, reconsidered the ancient ideathat each different kind of element is composed of a differentkind of atom (Figure 5.4).

Dalton imagined that all atoms were like small spheres butthat they could have different properties. They might vary in size,mass, or colour. Figure 5.5 shows how Dalton imagined atomswould look. Dalton used the following theory to explain thenature of matter:

• All matter is made of small, indivisible particles calledatoms.

• All the atoms of an element are identical in properties suchas size and mass.

• Atoms of different elements have different properties.

• Atoms of different elements can combine in specific ways toform new substances.

Dalton also devised a series of element symbols to represent theatoms of different elements. These symbols are shown in hisTable of Elements from 1808 (Figure 5.6). The small roundsymbols were meant to resemble atoms.

171The periodic table organizes elements by patterns in properties and atomic structure.

Figure 5.4 Science teacher andresearcher John Dalton

Figure 5.5 John Dalton suggested that atoms were like small spheres.Each element, he proposed, had a unique type of atom with aparticular mass.

Figure 5.6 John Dalton devised a set of elementsymbols to improve communication betweenscientists.

Suggested STSE Activity •B11 Quick Lab on page 176

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During Reading Atoms Are Composed of Smaller ParticlesJ. J. Thomson (1856–1940), an English physicist, researched theidea that atoms might be made from a combination of particles. Heexperimented with electric currents in glass tubes called cathoderay tubes (Figure 5.7).Using the tubes, he wasable to cause non-radioactive atomsto produce streams ofnegatively chargedparticles, later namedelectrons. Figure 5.8shows how the cathoderay tube worked.

Electrons are now understood to be negatively chargedparticles in atoms. Because all of the elements that Thomsontested in his cathode ray tube produced electrons, he reasoned thatatoms of all elements must contain electrons.

In 1897, Thomson proposed a revolutionary new model foratoms, in which each atom was composed of smaller particles.Because Thomson had detected negatively charged particles, hereasoned that atoms, which have no overall electric charge, mustalso contain positive charges. A diagram of Thomson’s model isshown in Figure 5.9. It depicts the atom as a positive sphere withnegative electrons scattered throughout it.

172 UNIT B Atoms, Elements, and Compounds

Figure 5.9 Thomson’s model of theatom.

Fixing Up UnderstandingUsing Illustrations

Make a note of the sentencesor paragraphs that you do notunderstand. Look at the figureson that page and the pagesbefore and after it. Reread eachsentence or paragraph, andconnect the words and ideas tothe illustrations. How do theillustrations help you tounderstand the words?

electricity source

magnet

electricalcondenserplatesFigure 5.8 In a cathode ray tube,

the heated metal at one end of thegas-filled tube sends out a stream ofelectrons.

Figure 5.7 J.J. Thomson used a simple cathode raytube like this one.

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The Discovery of the Atomic NucleusNew Zealand-born scientist Ernest Rutherford (1891–1937)tested Thomson’s model of the atom while working in England.Rutherford conducted an experiment in which he shot positivelycharged particles at a very thin foil of pure gold. Rutherfordobtained the stream of positive particles from a radioactivesubstance, which he placed in a lead block with a tiny hole. Out ofthe hole escaped a stream of these particles, which Rutherforddirected at the gold foil.

In the experiment, as shown in Figure 5.10, most of the high-speed positive particles went right through the foil. However,about 1 in 10 000 positive particles bounced back from the foil asif it had been deflected by something very massive and positivelycharged. Rutherford had discovered the nucleus, the centre of theatom. This tiny positively charged part of the atom also containsmost of the atom’s mass. He calculated that the size of the nucleuscompared to the rest of the atom was like the size of a single greenpea compared to that of an entire football field!

Based on his gold foil experiment, Rutherford revised the atomicmodel using his prediction that all atoms everywhere contain anucleus (Figure 5.11). His model was like Thomson’s except that allof the atom’s positive charge and most of the atom’s mass wereconcentrated at a tiny point in the centre. The electrons surroundedthe nucleus and occupied most of the atom’s volume, but theycontained only a small fraction of the atom’s total mass.

Inside the NucleusJames Chadwick (1891–1974), Rutherford’s student, refined theconcept of the nucleus. Chadwick discovered that the nucleuscontains neutral particles as well as positively charged particles.The neutral particles in the nucleus of the atom are calledneutrons. The positively charged particles in the atom are calledprotons. Each neutron in an atom has about the same mass aseach proton in the same atom, but the neutron carries noelectrical charge.

173The periodic table organizes elements by patterns in properties and atomic structure.

gold foil

high-speed particles

gold foil

high-speed particles

nucleus

atom

(a) prediction (b) evidence (c) new model

Figure 5.10 (a) Rutherford predictedthat if nothing blocked the way ofhigh-speed particles shot at a pieceof gold foil, then all the particleswould pass through the foil. (b) Thedata showed that somethingmassive blocked a few of theparticles. (c) Rutherford revised theatomic model to include thenucleus.

Figure 5.11 Rutherford’s modeldepicted the atom as a tiny yetmassive point of positive chargesurrounded by electrons.

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Electrons Exist in Energy LevelsDanish physicist Niels Bohr (1885–1962) studied the propertiesof electrons in atoms and, along with other researchers,transformed Rutherford’s model into one of the models that areused today (Figure 5.12). A simplified version of this model thatshows how electrons are arranged in the elements hydrogen andmagnesium is given in Figure 5.13. Although some of the featuresshown here, such as the pairing up of electrons, were discoveredafter Bohr did his work, this kind of illustration has come to beknown as a Bohr model, or Bohr diagram.

Bohr suggested that electrons surround the nucleus in specificenergy levels, called shells. He discovered that electrons jumpbetween these shells by gaining or losing energy. Each shell cancontain only a specific number of electrons. The maximum numberof electrons that can exist in each of the first three shells is two,eight, and eight. Many people still use this model to describe theparticles that make up the atom.

The Quantum Mechanical ModelThe most advanced and accurate model of the atom, and the onein use today by physicists and chemists, is called the quantummechanical model (Figure 5.14). In this model, electrons do notexist as tiny points inside an atom. Electrons exist in specificenergy levels, but they surround the positively charged nucleus ina form resembling a cloud.

174 UNIT B Atoms, Elements, and Compounds

nucleus

electron

hydrogen atom

Figure 5.13 Bohr diagrams like the ones shown here for hydrogen and magnesium areoften used to show the arrangement of electrons in atoms.

nucleus

electron shells

magnesium atom

electronFigure 5.12 Niels Bohr was only 28when he proposed his theory of theatom. In 1922, he won the NobelPrize in physics.

cloud of electrons

nucleus

Figure 5.14 The quantummechanical model of an atomdescribes a cloud of electronssurrounding the nucleus.

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Modern understanding of theproperties of matter is built on theinquiries of many different peoplefrom around the world working overthe ages. The alchemists, forexample, were people who tried touse magic and chemical changesto turn various substances intogold. In 1597, the Germanalchemist Andreas Libau publishedAlchemia, a book describing theachievements of alchemists. In it,Libau explained how to preparechemicals such as hydrochloricacid. Find out what else thealchemists discovered. Go toScienceSource to start yoursearch.

Take It Further

A Summary of the AtomAll elements are composed of atoms, and one atom is the smallestunit of any element. Although there are more than 100 differentelements, each with its own kind of atoms, the atoms themselvesare made of different kinds of smaller particles, called subatomicparticles. Three subatomic particles are protons, neutrons, andelectrons, and they have different properties.

One such property is relative mass. Relative mass comparesthe mass of an object to the mass of another object. An electron isthe least massive subatomic particle of the three subatomicparticles, so it is assigned a relative mass of 1. Compared to it, aproton has a relative mass of 1836, meaning that it is 1836 timesheavier than an electron. Compared to an electron, a neutron is1837 times heavier. This property of the particles is summarized inTable 5.1, along with electric charge and location within the atom.

175The periodic table organizes elements by patterns in properties and atomic structure.

Learning Checkpoint

1. (a) What is similar about the ancient and modern definitions of “elements”?

(b) What is different about the ancient and modern definitions of“elements”?

2. What evidence led J.J. Thomson to believe that atoms of all elementscontain electrons?

3. On what information did J.J. Thomson base his hypothesis that atomscontain positive particles?

4. Describe the experiment that showed that the atom has a dense, positivelycharged nucleus.

5. Describe three ways in which protons are different from electrons.

6. What are three features of a Bohr diagram?

Name Symbol Relative Mass Electric Charge Location

Proton p 1836 1+ nucleus

Neutron n 1837 0 nucleus

Electron e 1 1– in energy levels surrounding the nucleus

Table 5.1 Properties of Subatomic Particles

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176 UNIT B Atoms, Elements, and Compounds

B11 Quick Lab

It takes many scientists exploring differentpossibilities to develop a theory. The atomic theorytook shape only after many debates, novel ideas, andexperiments. Even today, scientists are makingdiscoveries that will add to our understanding of theatom.

PurposeTo learn about the contribution of particular scientiststo atomic theory

Procedure

1. Choose a scientist to research from the timelineshown in Figure 5.15 below.

2. ScienceSource Find information from twosources on the scientist that you have decided toresearch.

• Focus on one way that the scientist’s workhas shaped our understanding of the atom.

• Find out about at least one challenge thatthe scientist had to overcome.

Questions

3. Web 2.0 Summarize your findings as a Wiki, aslide presentation, a video, or a podcast, andpresent them to the class. For support, go toScienceSource.

4. How did your scientist’s contributions alter theprevious model of the atom?

5. How were your scientist’s ideas revised oncefurther research was done?

6. Do you think today’s atomic model will bechanged in future? Why or why not?

7. Why are collaboration and communicationbetween scientists necessary?

8. Ultimately, who do you think should get credit forthe current atomic theory? Justify your response.

Developing the Atomic Theory

1900s C.E.1800s C.E.1700s C.E.1600s C.E.410 B.C.E.DemocritusAristotle

Robert BoyleIsaac Newton

Joseph PriestlyAntoine LavoisierJoseph Louis Proust

John DaltonMichael FaradayJöns BerzeliusDmitri MendeleevWilliam CrookesHenry MoseleyJ.J. Thomson

Hantaro NagaokaHans GeigerErnest RutherfordHarriet BrooksHenri BecquerelMarie CurieNiels BohrMax PlanckJames ChadwickWerner HeisenbergLouis de BroglieRichard FeynmanMurray Gell-MannGerd BinningHeinrich Rohrer

Figure 5.15 Timeline of contributors to the atomic theory

STSE

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Key Concept Review1. How are atoms and elements related?

2. In your notebook, redraw and complete thefollowing table.

3. How does J. J. Thomson’s atomic modeldiffer from the model depicted by a Bohrdiagram?

4. What particles make up a cathode ray?

5. A statement is missing from the atomictheory given below. What is missing?

• Atoms of different elements havedifferent properties.

• All matter is made of small, indivisibleparticles called atoms.

• Atoms of different elements can combinein specific ways to form new substances.

Connect Your Understanding6. Atoms contain electrons, which are

negatively charged. Why are atomselectrically neutral?

7. Why do you think John Dalton used theGreek word for indivisible to describe atoms?

8. List two ways in which atoms of differentelements are different from one another.

9. J. J. Thomson’s discovery about electronswas an important step in the developmentof the atomic theory. Explain why.

10. History shows that many scientists makeimportant discoveries while they are stillstudents. Use one or more examples fromthis chapter to illustrate this point.

11. Use the following Bohr diagram of afluorine atom to complete this question.

Question 11

(a) How many electrons does a fluorineatom have?

(b) How many protons does a fluorine atomhave?

12. Create a flowchart that shows the atomicmodel at its different stages of development.Your flowchart should include:

• drawings of the different versions of theatomic model

• the names of the scientists whocontributed to each version of the atomicmodel

• labels to show how past versions of theatomic model are different from today’smodel

13. (a) Why do you think it took so long forpeople to accept the concept of atoms?

(b) Describe a discovery or experiment thatwould have made it easier for people tobelieve in atoms.

Reflection14. Consider an element that is important in

your life — for example, the element thatmakes up your watch or ring. How haveyour ideas about the composition of thiselement changed since completing thissection?

For more questions, go to ScienceSource.

177The periodic table organizes elements by patterns in properties and atomic structure.

Particle ChargeLocation inAtom

Proton

Electron

Neutron

Subatomic Particles

5.1 CHECK and REFLECT

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