5.3 Representative Groups

Reading Strategy Monitoring Your Understanding Copythe table below. As you read, record animportant fact about each element listed.

Key ConceptsWhy do the elementsin a group have similarproperties?

What are some propertiesof the A groups in theperiodic table?

Vocabulary◆ valence electron◆ alkali metals◆ alkaline earth

metals◆ halogens◆ noble gases

Why is hydrogen located on the left side of the periodic table withthe active metals? It is a nonmetal gas that seems to have more incommon with the nonmetals in Group 17. Hydrogen’s location isrelated to its electron configuration, not its properties.

Valence ElectronsDid you wonder why there are two numbering schemes on the periodictable in Figure 7? When the A groups are numbered from 1 through 8,they provide a useful reminder about the electron configurations ofthe elements in those groups. The number of an A group matches thenumber of valence electrons in an electron configuration for an ele-ment in that group. A valence electron is an electron that is in thehighest occupied energy level of an atom. These electrons play a keyrole in chemical reactions. Properties vary across a period because thenumber of valence electrons increases from left to right.

Elements in a group have similarproperties because they have the samenumber of valence electrons. These pro-perties will not be identical because thevalence electrons are in different energylevels. Valence electrons explain the loca-tion of hydrogen. Because hydrogen has asingle valence electron, it is grouped withother elements, such as lithium, that haveonly one valence electron.

Figure 14 Because hydrogen isflammable, it can be used as afuel in automobiles like this one.An engine that burns hydrogenhas a key advantage over anengine that burns gasoline. Only water is produced whenhydrogen burns.

a. ?

b. ?

c. ?

Element Important Fact

Magnesium

Aluminum

Chlorine

139

FOCUS

Objectives5.3.1 Relate the number of valence

electrons to groups in theperiodic table and to propertiesof elements in those groups.

5.3.2 Predict the reactivity of someelements based on theirlocations within a group.

5.3.3 Identify some properties ofcommon A group elements.

Build VocabularyConcept Map Have students constructa concept map with eight branches andtitle it Groups in the Periodic Table. Asstudents read, they can add the namesof groups to each branch, the group’snumber of valence electrons, elementsin the group, and some properties ofthese elements.

Reading StrategyPossible answers: a. Magnesium plays akey role in the production of sugar inplants. Mixtures of magnesium andother metals can be as strong as steel,but much lighter. b. Aluminum is themost abundant metal in Earth’s crust.Much less energy is needed to purifyrecycled aluminum than to extractaluminum from bauxite. c. Chlorine is a highly reactive, nonmetal gas that isused to kill bacteria in water.

INSTRUCT

Valence ElectronsIntegrate Space ScienceHydrogen exhibits metallic propertiesunder extreme conditions. Scientistshave theorized for decades that metallichydrogen exists in the core of planetssuch as Jupiter. Interior pressure onJupiter is millions of times greater thanthe pressure at the surface of Earth.Pressing hydrogen molecules extremelyclose together allows their electrons tomove relatively freely. Have students doresearch and write a paragraph thatcompares and contrasts the cores ofJupiter and Earth, according to thecurrent state of scientific knowledge. Verbal, Portfolio

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Reading Focus

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The Periodic Table 139

Print• Laboratory Manual, Investigation 5B• Guided Reading and Study Workbook

With Math Support, Section 5.3 • Transparencies, Section 5.3

Technology• iText, Section 5.3• Presentation Pro CD-ROM, Section 5.3• Go Online, Science News, Elements

Section Resources

Section 5.3

140 Chapter 5

The Alkali MetalsThe elements in Group 1A are called alkali metals. These metals havea single valence electron and are extremely reactive. Because they areso reactive, alkali metals are found in nature only in compounds. Themost familiar of these compounds is table salt—a compound ofsodium and chlorine (sodium chloride). Sodium chloride can beobtained through the evaporation of seawater or from large saltdeposits on the surface of Earth or underground.

Not all the elements in a group are equally reactive. Sodium is morereactive than lithium, potassium is more reactive than sodium, andrubidium is more reactive than potassium. The reactivity of alkalimetals increases from the top of Group 1A to the bottom.

Sodium is about as hard as cold butter and can be cut with a sharpknife, as shown in Figure 15A. Sodium melts at about 98°C and has alower density than water. A piece of sodium may be able to float onwater, but Figure 15B shows that it won’t be there for long. The sodiumreacts violently with water and releases enough energy to ignite thehydrogen gas that is produced. Sodium and potassium are storedunder oil to keep them from reacting with the oxygen and water vaporin air. Cesium is so reactive that it reacts with water at temperatures aslow as –115°C. Cesium is usually stored in a sealed glass tube contain-ing argon gas.

How many valence electrons does an alkalimetal have?

For: Articles on elements

Visit: PHSchool.com

Web Code: cce-1053

3

LiLithium

19

KPotassium

37

RbRubidium

11

NaSodium

55

CsCesium

87

FrFrancium

Group 1A

A

B

Figure 15 The element sodium is an alkalimetal A Unlike most metals, sodium is softenough to cut with a knife. B When sodiumreacts with water, enough energy is releasedto ignite the hydrogen that is produced.Predicting What happens when potassiumcomes in contact with water?

140 Chapter 5

The Alkali MetalsUse VisualsFigure 15 Ask students to study thetwo photos and the column of elementsin Figure 15. Ask, What properties ofsodium are shown in the photos?(Sodium is a soft solid at room tempera-ture with a metallic luster when firstexposed to air. Sodium is extremelyreactive and it reacts violently with waterto form hydrogen gas.) Point out tostudents that the reactivity of the alkalimetals increases from the top of thegroup to the bottom. Ask, Which alkalimetals are less reactive than cesiumbut more reactive than lithium?(Sodium, potassium, and rubidium)Visual, Logical

Build Science SkillsCommunicating Results Explain tostudents that Robert Bunsen and GustavKirchoff discovered cesium in 1840 andrubidium in 1841 by burning the sub-stances and observing the color of theflames. Have students research the originof the terms cesium and rubidium. Havestudents explain why these names areappropriate. (Cesium comes from the Latinword caesium, which means “heavenlyblue.” Rubidium comes from the Latinword rubidus, which means “dark red.”The names describe the colors of lightemitted when the elements are burned.)Logical

FYI Francium has been described as themost unstable element among the first103 elements. Its longest-lived isotope,francium-223, has a half-life of only 22minutes. The estimate is that there isonly about one ounce of francium onEarth at any moment.

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Section 5.3 (continued)

Science News provides studentswith current information onelements.

Customize for English Language Learners

Compare/Contrast ChartAfter students have read about Groups 1A and2A, create a chart on the board with the titleAlkali Metals vs. Alkaline Earth Metals. Separatethe chart into two columns labeled Similarities

and Differences. Ask for student responses to help you fill in the chart. After all correctanswers have been recorded, keep the chartdisplayed as a reference for students.

The Periodic Table 141

The Alkaline Earth MetalsThe elements in Group 2A are called alkaline earth metals. All alka-line earth metals have two valence electrons. Metals in Group 2A areharder than metals in Group 1A. The melting point of magnesium is650°C, which is much higher than the melting point of sodium—98°C.

Differences in reactivity among the alkaline earth metals areshown by the ways they react with water. Calcium, strontium, andbarium react easily with cold water. Magnesium will react with hotwater, but no change appears to occur when beryllium is added towater. Magnesium and calcium have essential biological functions andthey provide materials used in construction and transportation.

Magnesium Magnesium plays a key role in the process that usessunlight to produce sugar in plants like the one in Figure 16. The com-pound at the center of this process is chlorophyll (KLAWR uh fil), andat the center of chlorophyll is magnesium. A mixture of magnesiumand other metals can be as strong as steel, but much lighter. Reducingoverall mass without sacrificing strength is an important considera-tion in transportation. The frames of bicycles and backpacks oftencontain magnesium.

Calcium Your body needs calcium to keep your bones and teethstrong. Calcium carbonate—a compound of calcium, carbon, andoxygen—is the main ingredient in chalk, limestone, coral, and thepearl in Figure 16. Your toothpaste may contain the compoundcalcium carbonate because this hard substance can polish yourteeth. The plaster cast in Figure 16 contains calcium sulfate,which is a compound of calcium, sulfur, and oxygen.

4

BeBeryllium

12

MgMagnesium

20

CaCalcium

38

SrStrontium

56

BaBarium

88

RaRadium

Group 2A

Figure 16 Chlorophyll molecules in spinachcontain magnesium. An oyster shell and apearl are both made from calciumcarbonate. A plaster cast contains thecompound calcium sulfate.

Plaster cast

Oyster shell with pearl

Spinach plant

The Alkaline Earth Metals

Comparing Alkaline Earth MetalsPurpose Students will use observationsto distinguish three alkaline earthmetals.

Materials small samples of beryllium,magnesium, and calcium; cold and hotwater; 5 500-mL beakers

Safety Wear goggles and a lab apronand keep students at a safe distancefrom the reactions.

Procedure Fill each beaker with about100 mL of water. The water in three ofthe beakers should be cold. The water intwo of the beakers should be hot. Labelthe beakers of cold water A, B, and C.Label the beakers of hot water D and E.Tell students that you have samples ofberyllium, magnesium, and calcium forthem to identify. Put a small sample ofberyllium into beaker A. Use beakers Band C to repeat with magnesium andcalcium. Have students observe andrecord the results. Then, put samples ofberyllium and magnesium into beakersD and E, respectively. Have studentsobserve and record the results. Askstudents to use their observations toidentify the samples. (Calcium reactedwith cold water. Magnesium reacted withhot water. Beryllium did not react withcold or hot water. During the reactions,bubbles of hydrogen gas formed and thewater became cloudy.) You can invert atest tube of water over the calcium tomake the presence of a gas easier toobserve.

Expected Outcome Students willidentify samples based on their reactions(or lack of) with cold and hot water.Visual

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The Periodic Table 141

Plaster The mineral gypsum is heated toproduce a white powdery substance calledplaster of Paris. When water is added to plasterof Paris, heat is released as the plaster quicklyhardens. During the process, the plasterexpands by about 0.3–0.6%. A plaster cast isoften replaced by a sturdier, more lightweightfiberglass cast after the swelling around aninjury subsides.

Egyptians used plaster to join blocks ofstone in pyramids. Romans made plaster castsof Greek statues. In the 1700s, wooden housesin Paris were often covered in plaster to pro-tect against fire. This measure was taken inresponse to the destruction of London by firein 1666.

Facts and Figures

Answer to . . .

Figure 15 When potassium reactswith water, enough energy is releasedto ignite the hydrogen that is produced.

An alkali metal has onevalence electron.

The Boron Family Group 3A contains the metalloid boron, the well-known metal aluminum, and three less familiarmetals (gallium, indium, and thallium). All theseelements have three valence electrons.

Aluminum is the most abundant metalin EarthÕs crust. It is often found combined withoxygen in a mineral called bauxite (BAWKS eyet).Aluminum is less reactive than sodium and mag-nesium. It is strong, lightweight, malleable, and agood conductor of electric current.

More than 10 percent of the aluminum pro-duced is used as packaging. Some aluminum isused in window screens, window frames, and gut-ters. Parts of cars and airplanes are also made fromaluminum. People are encouraged to recycle alu-minum because the energy needed to purifyrecycled aluminum is only about 5 percent of theenergy needed to extract aluminum from bauxite.

A compound of boron, silicon, and oxygen isused to make a type of glass that does not shattereasily when it undergoes a rapid change in tem-perature. Glass that contains boron is used tomake laboratory glassware, such as the flasks inFigure 17. It is also used in cookware that can godirectly from the oven to the refrigerator.

The Carbon Family Group 4A contains a nonmetal (carbon), twometalloids (silicon and germanium), and twometals (tin and lead). Each of these elements hasfour valence electrons. Notice that the metallicnature of the elements increases from top tobottom within the group. In keeping with thistrend, germanium is a better conductor of electriccurrent than silicon.

Life on Earth would not exist without carbon.Except for water, most of the compounds in

your body contain carbon. Reactions that occurin the cells of your body are controlled by carboncompounds. Carbon and its compounds are dis-cussed in Chapter 9, Carbon Chemistry.

Silicon is the second most abundant elementin Earth’s crust. It is found as silicon dioxide inquartz rocks, sand, and glass. The clay used toproduce the pottery in Figure 18 contains siliconcompounds called silicates. Silicon carbide, acompound of silicon and carbon, is extremelyhard. Saw blades tipped with silicon carbide lastmany times longer than ordinary steel blades.

Which Group 3A element isa nonmetal?

31

GaGallium

81

TlThallium

13

Al Aluminum

5

BBoron

49

InIndium

Figure 17 These students are using flasksmade from glass that contains boron. Thistype of glass does not shatter as easily asglass without boron.

32

GeGermanium

82

PbLead

50

SnTin

14

SiSilicon

6

CCarbon

Figure 18 The clay used to make thispottery contains compounds calledsilicates. These compounds always containsilicon and oxygen. They usually containaluminum and often contain otherelements such as iron.

Group 3A

Group 4A

142 Chapter 5

142 Chapter 5

The Boron FamilyUse Community ResourcesInvite a representative from a communityrecycling program or a commercialrecycler to speak to your class about theimportance of recycling materials such asaluminum. Have students ask questionsabout the kinds of materials that arerecycled and how the recycling processis different for each of them. They mayalso ask how recycling has changed overthe past 10 years and what changes areexpected in the future. Interpersonal, Portfolio

The Carbon FamilyIntegrate MathTell students that silicon dioxide is themost abundant substance in Earth’scrust. It is, of course, a compound of theelements silicon and oxygen. Point outto students that only eight elementsmake up 98.5% of Earth’s crust: oxygen(46.6%), silicon (27.7%), aluminum(8.1%), iron (5.0%), calcium (3.6%),sodium (2.8%), potassium (2.6%), andmagnesium (2.1%). Have students workin pairs to create a circle graph thatshows this data. Remind students thatthey may need to combine some ele-ments under a category labeled Other.If students need help constructing acircle graph, you may want to referthem to the Math Skills in the Skillsand Reference Handbook at the end of the student text.Visual, Logical

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Section 5.3 (continued)

The Periodic Table 143

The Nitrogen FamilyGroup 5A contains two nonmetals (nitrogen andphosphorus), two metalloids (arsenic and anti-mony), and one metal (bismuth). Like the groupson either side of it, Group 5A includes elementswith a wide range of physical properties. Nitrogenis a nonmetal gas, phosphorus is a solid nonmetal,and bismuth is a dense metal. Despite their dif-ferences, all the elements in Group 5A have fivevalence electrons. Nitrogen and phosphorus arethe most important elements in Group 5A.

When air is cooled, the oxygen condensesbefore the nitrogen because nitrogen has a lowerboiling point than oxygen. Much of the nitrogenobtained from air is used to produce fertilizers,like the three shown in Figure 19. Besidesnitrogen, fertilizers often contain phosphorus.Your body uses compounds containing nitrogenand phosphorus to control reactions and releaseenergy from food.

Phosphorus exists as an element in severalforms with different properties. White phospho-rus is so reactive that it bursts into flame when itis in contact with oxygen. Red phosphorus is lessreactive and is used to make matches ignite.

The Oxygen FamilyGroup 6A has three nonmetals (oxygen, sulfur,and selenium), and two metalloids (tellurium andpolonium). All the elements in Group 6A have sixvalence electrons.

Oxygen is the most abundant element inEarth’s crust. Complex forms of life need oxygento stay alive because oxygen is used to release theenergy stored in food. Oxygen can be stored as aliquid under pressure in oxygen tanks. There mustbe no sparks or flames near an oxygen tankbecause materials that are flammable burn easilyin pure oxygen.

Ozone is another form of the element oxygen.At ground level, ozone can irritate your eyes andlungs. At upper levels of the atmosphere, ozoneabsorbs harmful radiation emitted by the sun.

Sulfur was one of the first elements to be dis-covered because it is found in large naturaldeposits like the one in Figure 20. The main useof sulfur is in the production of sulfuric acid, acompound of sulfur, hydrogen, and oxygen. Moresulfuric acid is produced in the United States thanany other chemical. About 65 percent of the sul-furic acid produced is used to make fertilizers.

34

SeSelenium

52

TeTellurium

84

PoPolonium

16

SSulfur

8

OOxygen

33

As Arsenic

51

Sb Antimony

83

BiBismuth

15

PPhosphorus

7

NNitrogen

Figure 20 Sulfur is often found innature in its elemental form—notcombined with other elements.Inferring What does this informationtell you about the reactivity of sulfur?

Figure 19 The composition of a fertilizervaries with its intended use. The numberson the bags of fertilizer are, from left toright, the relative amounts of nitrogen,phosphorus, and potassium.Analyzing Data Which type of fertilizercontains the most phosphorus?

Group 5A

Group 6A

The Nitrogen FamilyBuild Reading LiteracyIdentify Main Idea/Details Refer topage 98D in Chapter 4, which providesthe guidelines for identifying main ideasand details.

Guide students in applying this strategyto the text on page 143. Tell students to look for the main idea of eachparagraph and then list one or twosupporting details. If the paragraph hasno topic sentence, have students list twoimportant facts from the paragraph. Verbal, Portfolio

FYIOf the 18 elements essential for plantgrowth, nitrogen, phosphorus, andpotassium are most likely to be lacking.The numbers on the fertilizer packagesshown in Figure 19 represent percent N,percent P2O5, and percent K2O. (In thepast, oxides were often used as stan-dards for chemical comparisons.)Nitrogen and phosphorus are oftenpresent as nitrates and phosphates.

The Oxygen Family

Students may think that the air theybreathe is pure oxygen. Challenge thismisconception by reminding studentsthat air is a mixture of gases. Air is 78% nitrogen and 21% oxygen. Havestudents research what gases make up the remaining 1% of air. (Answersshould include carbon dioxide, watervapor, argon, and other noble gases.)Logical

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The Periodic Table 143

Answer to . . .

Figure 19 Tomato food

Figure 20 Sulfur is not highlyreactive under ordinary conditions.

Carbon

Phosphorus The element phosphorus has 10 forms, which are usually grouped as white,red, and black phosphorus for simplicity. Thewhite phosphorus forms are the least stable.

In 1680, Robert Boyle demonstrated thatphosphorus ignited by friction could be usedto light wooden splints that had been dippedin sulfur. There are two types of matches. In astrike-anywhere match, all of the required

ingredients (often phosphorus sulfide andpotassium chlorate) are in the match head. In a safety match, the ingredients are dividedbetween the match head and a rough strikingsurface on the side of the matchbox. A safetymatch can be lit only when the tip is drawnacross the striking surface, which containsphosphorus sulfide.

Facts and Figures

144 Chapter 5

The HalogensThe elements in Group 7A are called halogens. Each halogen has sevenvalence electrons. Figure 21 shows the range of physical propertiesamong the halogens. Fluorine and chlorine are gases, bromine is aliquid that evaporates quickly, and iodine is a solid that sublimes.

Despite their physical differences, the halogens have similarchemical properties. They are highly reactive nonmetals, with fluo-rine being the most reactive and chlorine a close second. Halogensreact easily with most metals. Figure 21 shows what happens whenheated steel wool is plunged into chlorine.

Recall that a fluorine compound is used to prevent tooth decay. Ifyou use pans with a nonstick coating to make omelets or muffins, youhave seen another use of a fluorine compound. Have you ever noticeda sharp smell when adding bleach to a load of clothes? The smell comesfrom a small amount of chlorine gas that is released from a chlorinecompound in the bleach. Chlorine is also used to kill bacteria in drink-ing water and swimming pools. The woman in Figure 21 is testing thelevel of chlorine in a swimming pool.

Your body needs iodine to keep your thyroid gland working prop-erly. This gland controls the speed at which reactions occur in yourbody. Seafood is a good source of iodine. At a time when fresh fish wasnot available in all parts of the United States, people began to addiodine compounds to table salt. Salt that contains such compounds iscalled iodized salt.

35

BrBromine

85

At Astatine

17

ClChlorine

9

FFluorine

53

IIodine

Group 7A

Chlorine Bromine Iodine

Figure 21 At room temperature, chlorine is a gas, bromine is aliquid, and iodine is a solid. Halogens react easily with metals, suchas the iron in steel wool. At a swimming pool, the chlorine contentmust be tested frequently. Applying Concepts What processcauses iodine vapor to collect in a flask of solid iodine?

Chlorine reacting with steel wool

144 Chapter 5

The HalogensBuild Reading LiteracyKWL Refer to page 124D in thischapter, which provides the guidelinesfor KWL (Know/Want to Know/Learned).

Teach this independent study skill as awhole-class exercise. 1. Draw a three-column KWL chart on the board forstudents to copy. 2. Have studentscomplete the Know column with facts,examples, and other information thatthey already know about the Group 7Aelements (the halogens). 3. Tell studentsto complete the Want to Know columnwith questions about the halogens. 4. Have students read p. 144 to learnmore about the halogens. As they read,have them note answers to their ques-tions in the Learned column, along withother facts, examples, and details theylearned. 5. Have students draw anInformation I Expect to Use box belowtheir KWL chart. Have them review theinformation in the Learned column and categorize the useful information in the box. Verbal

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Section 5.3 (continued)

Halogens The name astatine comes from the Greek astatos, meaning “unstable.”Astatine is a radioactive element whose moststable isotope, At-210, has a half-life of only8.1h. Astatine is the most metallic of thehalogens. It is usually classified as a metalloid,but is sometimes regarded as a nonmetal.

Because of its rarity and instability, astatine hasno practical uses.

Both bromine and iodine are volatile. Iodinecomes from the Greek ioeides, meaning “violetcolored.” It is named for the color of iodinevapor, not the color of its solid crystals, whichare dark gray.

Facts and Figures

Section 5.3 Assessment

Reviewing Concepts1. Explain why elements in a group have

similar properties.

2. What is the relationship between an alkalimetal’s location in Group 1A and its reactivity?

3. What element exists in almost everycompound in your body?

4. Which Group 5A elements are foundin fertilizer?

5. Which group of elements is the least reactive?

6. Why is hydrogen located in a group withreactive metals?

7. What biological function requires magnesium?

8. Why is aluminum recycled?

9. What is the main use of sulfur?

10. Why is chlorine added to drinking water?

Critical Thinking 11. Comparing and Contrasting In which

class of elements is there a greater range ofproperties, the metals or the nonmetals? Givean example to support your answer.

12. Making Generalizations What happens tothe reactivity of nonmetals within a groupfrom the top of the group to the bottom?

The Noble Gases The elements in Group 8A are called noble gases. Helium hastwo valence electrons. Each of the other noble gases has eightvalence electrons. The noble gases are colorless and odor-less and extremely unreactive. In Chapter 6, you will study therelationship between the electron configurations of the noblegases and their low reactivity.

It is not easy to discover a colorless, odorless gas. It is evenharder if the gas rarely reacts. Scientists discovered argon whenthey noticed that the density of nitrogen collected from air didnot match the density of nitrogen formed during chemicalchanges. In time, the scientists figured out that the “impurity” inatmospheric nitrogen was an unknown element.

An element that does not react easily with other elementscan be very useful. For example, during one stage in the processof making computer chips, pure silicon is heated in a furnace at1480�C. At this temperature, silicon reacts with both oxygen andnitrogen. So the heating must take place in an argon atmosphere.

Some light bulbs are filled with argon because the glowingfilament in the bulb will not react with argon as it would reactwith oxygen. Using argon increases the number of hours the bulbcan be lit before it burns out. All the noble gases except radonare used in “neon” lights like those shown in Figure 22.

The Periodic Table 145

Using Physical Properties In Section 2.2,three ways to use physical properties arediscussed. Find one example in Section 5.3that illustrates each use. If necessary, rereadpages 48 and 50.

36

KrKrypton

54

XeXenon

86

RnRadon

18

Ar Argon

10

NeNeon

2

HeHelium

Figure 22 When electric current passesthrough noble gases, they emit differentcolors. Helium emits pink, neon emitsorange-red, argon emits lavender, kryptonemits white, and xenon emits blue.

Group 8AThe Noble GasesBuild Science SkillsInferring Hold up an incandescentlight bulb and tell students that thefilament in the light bulb is made oftungsten, which emits light when it isheated to a high temperature. Explainthat a light bulb with a tungstenfilament contains small amounts ofgases, such as argon, that do not reacteasily. Ask, Why isn’t air used in thelight bulb? (The heated filament wouldburn in air, which contains oxygen.)Visual, Logical

ASSESSEvaluate UnderstandingWrite the names of A group elementsdiscussed in Section 3.3 on separateindex cards. Distribute one card to eachpair of students and have them add thefollowing information to the card: chem-ical symbol, atomic number, groupnumber and group name, number ofvalence electrons, physical state at roomtemperature, class of element (metal,nonmetal, or metalloid), and one or twoproperties of the element.

ReteachUse the segments of the periodic tablethat appear throughout the section toreview each A group. Emphasize howthe metallic properties of elementsincrease from top to bottom within agroup and how the number of valenceelectrons changes from left to rightacross the periodic table.

Examples of Identifying Materials:Distinguish halogens by their states andcolors at room temperature.

Examples of Choosing Materials: Using hard calcium carbonate to polishteeth; using argon to increase the life of a light bulb.

Examples of Separating Materials:Sodium chloride from seawater byevaporation; nitrogen from oxygenbased on their boiling points.

If your class subscribes toiText, use it to review key concepts inSection 5.3.

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8. The energy needed to purify recycled alum-inum is only about 5% of the energy needed to extract aluminum from bauxite.9. To produce sulfuric acid10. Chlorine is added to drinking water to kill bacteria.11. Accept all answers that are supported byreasonable arguments. Students may choosenonmetals and say that they display a greaterrange of physical properties and reactivity. 12. The reactivity of nonmetals decreases fromthe top to the bottom of a group.

Section 5.3 Assessment

1. They have similar properties because theyhave the same number of valence electrons.2. The reactivity of alkali metals increases fromthe top of Group 1A to the bottom.3. Carbon4. Nitrogen and phosphorus5. The noble gases (Group 8A)6. Hydrogen is placed with other elementsthat have a single valence electron. 7. The process that uses sunlight to producesugar in plants

Answer to . . .

Figure 21 Sublimation

The Periodic Table 145

Eating a variety of foods helps to ensure that allthe elements needed by your body are available.The required elements can be classified as major,lesser, or trace elements. An element is classifiedbased on its percentage by mass in the body.

The six major elements are hydrogen,oxygen, carbon, nitrogen, phosphorus, andcalcium. These six major elements account foralmost 99 percent of your body mass. Nearlyevery compound in your body contains carbonand hydrogen, and many contain oxygen too.The compounds that control all the chemicalchanges that take place in cells contain nitrogen.Calcium is essential for healthy bones and teeth.Phosphorus is found in your DNA and in themolecules that transfer energy within cells.

The lesser elements are iron, potassium,zinc, sodium, sulfur, chlorine, and magnesium.For each lesser element, there is a recommendedamount that needs to be taken in daily. Theseamounts vary from 15 milligrams for zinc to400 milligrams for magnesium. Lesser elementshelp your body build tissues and maintain othercell processes. For example, nerves and musclesrequire magnesium to function properly.

The trace elements are vanadium, chromium,molybdenum, manganese, cobalt, copper, boron,tin, silicon, selenium, fluorine, and iodine. Thequantities required are tiny, but trace elementsperform important functions. For example, redblood cells would not mature without cobalt.

Elemental Friends and FoesSome elements are essential for your health, and some areextremely harmful. You need to obtain the right amountsof the twenty-five essential elements through a balanceddiet, and to reduce your exposure to the harmful elements.

Essential Elements of the Human Body

Major elements (approximately 98.6 percent of total)

Lesser elements (approximately 1.4 percent of total)

Trace elements (less than 0.01 percent of total)

Nonessential elements

Se

Si P S Cl

C N O F

Sn I50

Tin

53

Iodine

34

Selenium

14

Silicon

15

Phosphorus

16

Sulfur

17

Chlorine

6

Carbon

7

Nitrogen

8

Oxygen

9

Fluorine

H

Na Mg

K Ca V Cr Mn Fe Co Cu Zn

Mo42

Molybdenum

24

Chromium

25

Manganese

26

Iron

27

Cobalt

29

Copper

30

Zinc

23

Vanadium

20

Calcium

19

Potassium

11

Sodium

12

Magnesium

1

Hydrogen

146 Chapter 5

B5

Boron

146 Chapter 5

Elemental Friends and FoesBackgroundAll packaged and processed food sold in the United States displays a label with nutritional information. The label is designed to be easy to read and toenable consumers to quickly find theinformation they need to makeappropriate food choices.

The label contains a Daily Valuepercentage (%DV) for calorie-containingnutrients such as fats and carbohydrates,as well as for cholesterol, vitamins, andminerals. For vitamins and minerals, the%DV is based on the Reference DailyIntake (RDI) values established by theU.S. Food and Drug Administration.

On current food labeling, the %DV forvitamins and minerals is the percentageof RDI available in a single serving. Forexample, the RDI for magnesium is 400milligrams. If a single serving of a certainbrand of cereal contains 40 milligrams ofmagnesium, the %DV for magnesium islisted as 10%.

It is likely that the number of elementsclassified as essential will grow alongwith advancements in understanding of nutrition and the human body. Forexample, nickel is an essential element in some species, but has not yet beendetermined to be essential in humans.

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Essential elementsThe 25 elements essential to the human bodyare generally ingested as part of compoundsfound in food. The roles and functions of afew of these elements are shown here.

OxygenThis is the most abundantelement in your body.

The most importantfunction of oxygen is tohelp release the energystored in foods. Youabsorb oxygen fromthe air you breathe.

O8

Oxygen

VanadiumVanadium can help controlblood sugar levels. It alsoplays an important role in

the formation ofbones and teeth.

Vanadium can befound in blackpeppercorns.

V23

Vanadium

IodineIodine isrequired

in small amountsfor the productionof thyroxine by thethyroid gland. Thyroxinecontrols the rate of all chemical processes in the body. Fresh fish is a good source of iodine.

I53

Iodine

HydrogenHydrogen is found infoods and in water,

which accounts for morethan 60 percent of bodymass. Chemical reactionsin cells take place in water.

H1

Hydrogen

CarbonCarbon is thesecond most

common element inthe human body. It isessential to life, because it ispresent in almost every compoundin the body. If a food containscarbohydrates, proteins, or fats,it contains carbon compounds.

C6

Carbon

PotassiumPotassium is essential to muscle and nerve

function, and helps keepthe body’s fluids inbalance. It also stimulatesthe kidneys to removebody wastes. Potassiumcan be found in fruitand dairy products.

K19

Potassium

IronIron is a veryimportant trace

element because it ispart of hemoglobin.This compound transportsoxygen through the bloodto every cell in the body.Meat, fish, and leafy greenvegetables, such as spinach,are good sources of iron.

Fe26

Iron

RED SNAPPER

WATER

BLACK PEPPERCORNS

PASTA

CHEESES

GRAPES

SPINACH

The Periodic Table 147

Build Science Skills

Drawing Conclusions

Purpose Students identify elements whose compounds are used to fortify foods.

Materials empty containers of fortifiedfood products such as milk, juice, salt,and bread

Class Time 15 minutes

Procedure Explain that sometimescompounds of essential elements areadded to foods so that people will havea sufficient amount of these elements in their diets. Tell students that to fortifymeans to enrich or increase the nutrientvalue of food. Pass the empty containersaround. Have students look at the labels.Ask, Which essential elements havebeen added to the foods? (Answersmay include calcium, iron, and iodine.)Why are these elements essential?(Students’ answers should includeinformation from pp. 146 and 147.)

Expected Outcome Students discoverthat compounds of calcium, iron, andiodine are likely to be added when food is fortified.Logical, Visual

L2

The Periodic Table 147

Some elements should be avoided completely.These foes include metals, such as lead andmercury, and the metalloid arsenic. Harmfulelements may enter the body in water, air, orfood. Inside the body, nonessential elementsmay compete with essential elements anddisrupt cell functions. Large amounts ofharmful elements can stress the body’s normalmethods for eliminating toxins. Harmfulelements can build up in body tissues.

For decades lead was used in paint and ingasoline to improve engine performance. Theseproducts are now lead free. The use of mercuryin thermometers has been reduced and arseniccompounds are no longer used in pesticides.

Many trace elements are only helpful ifingested in the small recommended amounts.Larger quantities of most trace elements canbe harmful, as can larger-than-requiredamounts of lesser elements, such as sodium.

LeadToo much lead inthe bloodstream

can lead to organdamage and learningdifficulties, especially inyoung children. For thisreason, the use of lead-based paint was bannedin 1978. Children maystill be a risk if they livein an older house withlayers of peeling paint.

ArsenicArsenic has been known as apoison for centuries, yet

compounds of arsenic were usedto treat some diseases before thediscovery of penicillin. Arseniccompounds are no longer used toprotect crops from insect pests or topreserve wood. Drinking water canabsorb arsenic as it flows over rocks.But the amount of arsenic in drinkingwater is limited by law to less than50 micrograms in each liter of water.

As33

Arsenic

Pb82

Lead

Arsenicpesticide

MercuryMercury is used in numerous

industrial processes,but it is toxic andcan damage thebrain and nervoussystem. For thisreason it should behandled with greatcare. It is particularlyimportant to avoidinhaling mercury vapor.

80

MercuryHg

148 Chapter 5

Harmful Elements

148 Chapter 5

Build Science SkillsMaking Judgments Assign as aresearch topic one of the harmfulelements—mercury, lead, or arsenic—to each student. Have students use theInternet or the library to find two or threerecent newspaper or magazine articlesrelated to the harmful effects of exposureto the element. Ask students to write aparagraph summarizing each article, and then write a paragraph explainingwhether or not they think that moresteps should be taken to protect peoplefrom exposure to the element.

Have a class discussion or debate inwhich students present their researchfindings and their viewpoints. Ask, Whatare the harmful effects of the elementyou researched? (Sample answer:Exposure to lead can damage the centralnervous system, kidneys, and digestivesystem.) What precautions for the useof this element are currently in place?(Sample answer: Lead is no longer allowedas a gasoline additive; regulations prohibitrenting an apartment or house with peel-ing and flaking lead paint to families withyoung children; and lead levels in drinkingwater have been reduced.) Are theseprecautions sufficient? Explain youranswer. (Sample answer: Yes, becausethese regulations have helped significantlyreduce the number of children sufferingfrom lead exposure. According to theCenters for Disease Control, this numberdecreased about 80% between the late1970s and mid-1990s. An EPA documentdated December 2002 reports that in1978 nearly three to four million childrenwere affected. In the 1990s, the numberdeclined to about 890,000.)Interpersonal, Group

L2

(continued)

■ Choose a trace element other than vanadium,selenium, or iodine. Find out which foods aregood sources of the element.Write a paragraph explaininghow your diet meets or couldbe adjusted to meet yourneed for this element.

■ Take a Discovery Channel Video Field Trip by watching “You Are WhatYou Eat.”

Going Further

ZincThis trace element can befound in almost every cell of

your body. Among other things, ithelps to support a healthy immunesystem. Beef is a good source ofzinc, but no more than 40 milli-grams of zinc should be takendaily. Too much zinc can causeanemia by reducing iron uptake.

SodiumIt is hard to avoid sodium inyour diet because table salt

contains a sodium compound(sodium chloride). Everybodyneeds some sodium each day tomaintain water balance and nervefunction. But too much sodium(more than 3 grams daily) cancause high blood pressure.

Zn30

Zinc

Na11

Sodium

SeleniumThis trace element helps to maintain ahealthy immune system. Brazil nuts are a

good source of selenium, as are fruits andvegetables. While selenium supplements maybe useful for some people, no more than 400 micrograms should be taken daily. Toomuch selenium can cause nerve damage.

Se34

Selenium

BEEF

Video Field Trip

BRAZIL NUTS

TABLE SALT

Too much of a good thingMany of the elementsfound in your body areneeded in only very smallamounts. Too much canoften be harmful. The Food and DrugAdministration providesguidelines on the safe dailyquantities to take throughfood or supplements.

The Periodic Table 149

Integrate HealthA person’s diet must contain somesodium compounds so that importantfunctions such as maintenance ofappropriate water levels within cells andtransmission of nerve impulses will occur.However, the level of sodium compoundsin food (especially in processed foods) isso high that most people’s intake ofsodium compounds exceeds requiredlevels. Have students research thepossible adverse health effects of anexcess of sodium in the body.Verbal

Going FurtherChromium: black pepper, broccoli,asparagus, mushrooms, liver, raisins,nuts, brewer’s yeast; molybdenum:legumes, leafy vegetables, grains;manganese: nuts, oatmeal, ginger, rice; cobalt: meat, dairy products, greenleafy vegetables; silicon: whole grains,liver, red meat. (Tin exists in sufficientquantities in water, food, and air.)Verbal, Portfolio

L2

The Periodic Table 149

After students have viewed the Video Field Trip, ask them the following questions: How did someancient civilizations preserve their dead? (Byembalming, or using preservatives) What are twotypes of information that scientists discover by examining mummies? (Student answers mayinclude age, gender, diet, and cause of death.) Whatevidence led forensic scientists to suspectTutankhamen, also known as King Tut, might

not have died from natural causes? (X-raysshowed bone damage to his skull probably caused by a blow to his head.) List features that scientistsfound in the bones of mummies in Chile.(Student answers may include bone deformationaround the ear, bones with significant nitrogendeposits, and strong and healthy teeth.) What doforensic scientists suspect to be the causes ofthe features found in these mummies? (Fishing in cold waters could have caused the growth aroundthe ears. Nitrogen deposits in their bones could havebeen caused by a diet rich in seafood. A diet of fishcould explain the strong, healthy teeth.)

Video Field Trip

You Are What You Eat