Mixed Groups

7f)w You'll Learn It's Important Review Vocabulary • Distinguish among , The elements in mixed groups affect substance: element or compound , and . your life every day, because they are that cannot be broken down into • Describe the nature of allotropes. in everything from the computer you simpler components • Recognize the significance of use to the air you breathe. differences in structure New Vocabulary in . • • Understand the importance • allotrope of synthetic elements. • semiconductor • transuranium element

Properties of Metalloids The Can an element be a and a ? In a sense, some 5 elements called metalloids are. Metalloids share unusual charac- B teristics. Metalloids can form ionic and covalent bonds with 13 other elements and can have metallic and nonmetallic proper- AI r ties. Some metalloids can conduct electricity better than most 31 nonmetals, but not as well as some metals, giving them the Ga name semiconductor. '>\lith the exception of aluminum, the 49 metalloids are the elements in the that are located In along the stair-step line. The mixed groups-13, 14, 15, 16, and " /J 81 II I 17-contain metals, nonmetals, and metalloids. 'HI TI I I The Figure 16 Aluminum is used fre- Boron, a metalloid, is the first element in Group 13. Ifyou look quently in the construction of air- around your home, you might find two compounds ofboron. One planes because it is light and strong. of these is borax, which is used in some laundry products to soften water. The other is boric acid, a mild antiseptic. Boron also is used as a grinding material and as boranes, which are compounds used for jet and rocket fuel. Aluminum, a metal in Group 13, is the most abundant metal in Earth's crust. It is used in soft- drink cans, foil wrap, cooking pans, and as siding. Aluminum is strong and light and is used in the con- struction of airplanes such as the one in Figure 16.

584 CHAPTER 19 Elements and Their Properties Figure 17 Elements in Group 14 have many uses.

Silicon is used to make the chips that allow this computer to run.

The These tin cans are made of steel Each element in Group 14, the carbon family, has four elec- with a tin coating. trons in its outer energy level, but this is where much of the sim- ilarity ends. Carbon is a nonmetal, and are metalloids, and tin and lead are metals. Carbon occurs as an ele- ment in coal and as a compound in oil, natural gas, and foods. Carbon in these materials can combine with OA)'gen to produce carbon dioxide, CO2, In the presence of sunlight, plants utilize CO2 to make food. Carbon compounds, many of which are essential to life, can be found in you and all around you. All organic compounds contain carbon, but not all carbon com- pounds are organic. Silicon is second only to oxygen in abundance in Earth's The Carbon Group crust. Most silicon is found in sand, Si02, and almost all rocks 6 and soil. The of silicon dioxide is similar to the ( structure of . Silicon occurs as two allotropes. 14 Allotropes, which are different forms of the same element, have 51 different molecular structures. One allotrope of silicon is a hard, 32 gray substance, and the other is a brown powder. Ge 50 Whatareallotropes? 5n \' \ I / II Silicon is the main component in semiconductors-ele- \ 82 I l I ments that conduct an electric current under certain conditions. Pb Many of the electronics that you use every day, like the computer in Figure 17, need semiconductors to run. Germanium, the other metalloid in the carbon group, is used along with silicon in mak- ing semiconductors. Tin is used to coat other metals to prevent corrosion, like the tin cans in Figure 17. Tin also is combined with other metals to produce bronze and pewter. Lead was used widely in paint at one time, but because it is toxic, lead no longer is used.

SECTION 3 Mixed Groups 585 Allotropes of Carbon What do the diamond in a diamond ring and the graphite in your pencil have in common? They are both carbon. Diamond, graphite, and buckminsterfullerene, shown in Figure 18, are allotropes of an element. '\ A diamond is clear and extremely hard. In a diamond, each Scienc line carbon is bonded to four other carbon at the ver- tices, or corner points, of a tetrahedron. In turn, many tetra- Topic: Buckminsterfullerene hedrons join together to form a giant molecule in which the Visit gpscience.com for Web links atoms are held tightly in a strong crystalline structure. This to information about this structure accounts for the hardness of diamond. compound. Graphite is a black powder that consists of hexagonal layers Activity Research this com- of carbon atoms. In the hexagons, each carbon atom is bonded pound and describe some of the to three other carbon atoms. The fourth electron of each atom qualities that make it unique. is bonded weakly to the layer next to it. This structure allows the layers to slide easily past one another, making graphite an excel- lent lubricant. In the mid-1980s, a new allotrope of carbon called buckminsterfullerene was discovered. This soccer-ball- shaped molecule, informally called a buckyball, was named after the architect-engineer R. Buckminster Fuller, who designed structures with similar shapes. In 1991, scientists were able to use the buckyballs to synthe- size extremely thin, graphitelike tubes. These tubes, called nan- otubes, are about 1 billionth of a meter in diameter. That means Figure 18 Three allotropes you could stack tens of thousands of nanotubes just to get the of carbon are depicted here. thickness of one piece ofpaper. Nanotubes might be llsed some- Identify the geometric shapes day to make computers that are smaller and faster and to make that make up each allotrope. strong building materials.

.. Graphite Diamond Buckminsterfullerene

586 CHAPTER 19 Elements and Their Properties The Nitrogen Group

The nitrogen family makes up Group 15. Each element has 7 five electrons in its outer energy leveL These elements tend to The Nitrogen N Group share electrons and to form covalent compounds with other ele- 15 ments. Nitrogen often is used to make nitrates (which are com- p

pounds that contain the nitrate ion, N03-) and ammonia, NH3' 33 both ofwhich are used in fertilizers. Nitrogen is the fourth most As """ abundant element in your body. Each breath you take is about 51 80 percent gaseous nitrogen in the form of diatomic molecules, Sb I/J N . Yet you and other animals and plants can't use nitrogen in ,I I 83 / I 2 'J / -1 Bi its diatomic form. The nitrogen must be combined into com- - pounds, such as amino acids.

Use Circle Graphs

CRUST ELEMENTS OAI'gen, the predominant element in Earth's crust, makes up approximately 46.6 percent of the crust. If you were to show this information on a circle graph, how many degrees would be used to represent oxygen? IIlImiID known values and unknown values Identify the known values: OAl'gen is approximately 46.6% circle contains 3600 Identify the unknown values: how many degrees OAl'gen represents

E!lD the problem 46.6 _ x 100 3600

x = = 167.760 or 1680 ImD the answer Does your answer seem reasonable? Check your answer by dividing your answer by 3,60, then multiplying by 100.

1. The percentages of remaining elelnents in Earth's crust are: silicon, 27.7; alUlninum, 8.1; iron, 5.0; calcium, 3.6; sodium, 2.8; potasSiUIl1, 2.6; magnesium, 2.1; and other elements, 1.5. Illustrate in a circle graph. 2. The approximate percentages of the elements in the human body are: oxygen, 65; carbon, 18; , 10; nitrogen, 3; calcium, 2; and other elements which account for 2. lIlustrate these percentages in a circle graph. For more practice problems, go to page 834, and visit gpscience.com/extra_problems.

SECTION 3 Mixed Groups 587 Uses of the Nitrogen Group is a nonmetal that has three allotropes. Phosphorous compounds can be used for many things from water softeners to fertilizers, match heads, and even in fine china. Antimony is a metalloid, and bismuth is a metal. Both elements are used with other metals to lower their melting points. Because of this property, the metal in automatic fire-sprinkler heads contains bismuth.

The Oxygen 8 Why is bismuth used in fire-sprinkler heads? Group 0 16 l- S The Oxygen Group 34 Se Group 16 on the periodic table is the oAi'gen group. You can live for only a short time without oA)'gen, which makes up about S2 Te i/jLL 21 percent of air. Oxygen, a nonmetal, exists in the air as I rr /1 r diatomic molecules, 02' During electrical storms, some oA)'gen IJ IT 84 r ,- Po molecules, 02' change into ozone molecules, 03' Oxygen also has several uses in compound form, including the one shown at left in Figure 19. Nearly all living things on Earth need 02 for respiration. Living things also depend on a layer of03 around Earth for pro- tection from some of the Sun's radiation. The second element in the OA)'gen group is sulfur. Sulfur is a nonmetal that exists in several allotropic forms. It exists as different-shaped and as a noncrystalline solid. Sulfur combines with metals to form sulfides of such distinctive colors that they are used as 'pigments in paints. The nonmetal selenium and two metalloids-tellurium and polonium-are the other Group 16 elements. Selenium is the most common of these three. This element is one of several that you need in trace amounts in your diet. Many multivitamins contain this nonmetal as an ingredient. But selenium is toxic if too much of it gets into your system. Selenium also is used in Figure 19 Group 16 com- photocopiers like the one in Figure 19. pounds have a variety of uses.

Solutions of hydrogen peroxide,

H202, are used to clean minor wounds.

Selenium is used in xerography to make photocopies.

588 CHAPTER 19 Elements and Their Properties Figure 20 The used in smoke detectors is a that has saved lives. Battery ---+---'"

+-;:=:-__ £i'---T--- Alarm --r--Smoke Smoke detector

Smoke particle -_,.." Synthetic Elements If you made something that always fell apart, you might think you were not successful. However, nuclear scientists are learning to do just that. By smashing existing elements with par- ticles accelerated in a heavy ion accelerator, they have been suc"- cessful in creating elements not typically found on Earth. Except for 43 and 61, each synthetic element has more than 92 protons. Bombarding with can make , element 93. Half of the synthesized atoms of neptunium disin- tegrate in about two days. This may not sound useful, but when The Transuranium Elements r - neptunium atoms disintegrate, they form . This highly toxic element has been produced in control rods of nuclear reactors and is used in bombs. Plutonium also can be changed to americium, element 95. This element is used in home smoke detectors such as the one in Figure 20. In smoke , detectors, a small amount of americium emits charged particles. An electric plate in the smoke detector attracts some of these charged particles. When a lot of smoke is in the air, it interferes with the electric current, which immediately sets off the alarm in the smoke detector.

Transuranium Elements Elements having more than 92 pro- tons, the of uranium, are called transuranium elements. These elements do not belong exclusively to the metal, nonmetal, or metalloid group. These are the elements toward the bottom of the periodic table. Some are in the series, and some are on the bottom row of the main periodic table. All of the transuranium elements are synthetic and unstable, and many of them disintegrate quickly.

SECTION 3 Mixed Groups 589 NATIONAL OGEOGRAPHIC VISUALIZING THE DISCOVERY OF ELEMENTS Why make elements? Figure 21 shows when some of the elements were discovered throughout history. The processes used to discover these elements have varied widely. The most recently discovered elements are synthetic. By studying how the synthesized elements form and disintegrate, you can gain an understanding of the forces holding the nucleus together. When these atoms disintegrate, they are said to be radioactive. Scienc Radioactive elements can be useful. For example, technetium's Topic: Synthetic Elements radioactivity makes it ideal for many medical applications. At this Visit gpscience.com for Web links to information and an online time, many of the synthetic elements last only small fractions of update about synthetic elements. seconds after they are constructed and can be made only in small amounts. However, the value of applications that might be dis- Activity Find what some of the latest developments are in covered easily could offset their costs. synthetic elements. Collect infor- mation on the one that interests Seeking Stability Element 114, discovered in 1999, appears to you most and explain what you be much more stable than most synthetic elements of its size. It think is the most intriguing prop- lasted for 30 s before it broke apart. This may not seem like long, erty of this element. but it lasts 100,000 times longer than an atom of element 112. Perhaps this special combination of 114 protons and 175 neutrons allows the nucleus to hold together despite the enormous repul- sion between the protons. In the 1960s, scientists theorized that stable synthetic elements exist. Finding one might help scientists understand how the forces inside the atom work. Perhaps someday you'll read about some of the everyday uses this discovery has brought.

Summary Self Check Properties of Metalloids 1. Explain why Groups 14 and 15 are better representa- • Metalloids are elements that can form ionic tives of mixed groups than Groups 13 and Group 16. and covalent bonds with other elements and 2. Describe how allotropes of silicon differ in appearance. can have metallic and nonmetallic properties. 3. Explain how an element is classified as a transuranium Carbon Group element. • The elements in Group 14 have four electrons 4. Describe what type of structure a diamond has. How in their outer energy levels. would you build a model of this? Nitrogen Group 5. Think Critically Graphite and a diamond are both • The elements in Group 15 tend to share elec- made of the element carbon. Why is graphite a lubri- trons and form covalent bonds. cant and diamond the hardest gem known? Synthetic Elements • Synthetic elements are elements that are not 6. Calculate Element 114 lasted 30s before falling apart. typically found on Earth. It lasted 100,000 times longer than element 112. How • By synthesizing elements, scientists may under- long did element 112 last? stand how the forces inside the atom work.

\ Selene gpscience.com/seICcheck_quiz SECTION 3 Mixed Groups 591 StipperY Carbon

Goals Real-World • Make a model that Often, a lubricant is needed when two metals will demonstrate the touch each other. For example, a sticky lock molecular structure of sometimes works better with the addition of graphite. a small amount of graphite. What gives this • Compare and contrast allotrope of carbon the slippery property of a the strength of the lubricant? Why do certain arrangements of different bonds in atoms in a material cause the material to feel graphite. slippery? • Infer the relationship between bonding and Form a physical properties. Based on your understanding of how carbon Possible Materials atoms bond, form a hypothesis about the rela- thin spaghetti tionship of graphite's molecular structure to small gumdrops its physical properties. thin polystyrene sheets flat cardboard scissors Safety Precautions E:::.. 'Wi n IE .... Use care when working with scissors and uncooked spaghetti.

592 CHAPTER 19 Elements and Their Properties Test Your Make A Plan 1. As a group, agree upon a logical hypothesis statement. 2. As a group, sequence and list the steps you need to take to test your hypothesis. Be specific, describing exactly what you will do at each step to make a model of the types of bonding present in graphite. 3. Remember from Figure 18 that graphite consists of rings of six bonded in a flat hexagon. These rings are bonded to each other. In addition, the flat rings in one layer are weakly attached to other flat layers. 4. list possible materials you plan to use. 5. Read over the experiment to make sure that all steps are in logical order. 6. Will your model be constructed with materials that show weak and strong attractions? Follow Your Plan 1. Make sure your teacher approves your plan before you start. 2. Have you selected materials to use in your model that demonstrate weak and strong attractions? Carry out the experiment as planned. 3. Once your model has been constructed, list any observations that you make and include a sketch in your Science Journal.

Analyze Your 1. Compare your model with designs and results of other groups. 2. How does your model illustrate two types of attractions found in the graphite structure? 3. How does the bonding of graphite that you explored in the lab explain graphite's lubricating properties? Write your answer in your Science Journal.

Conclude and Apply-- 1. Describe the results you obtained from your ommunicating experiment. Did the results support your .. . hypothesis? Explain to a friend why graphite makes a 2. Describe why graphite makes a good lubricant. good lubricant and how the two types of 3. Explain what kinds of bonds you think a bonds make a difference. diamond has.

LAB 593

. . •

Section 1 Metals 4. The noble gases, Group 18, are elements whose properties and uses"are related to 1. A typical metal is a hard, shiny solid their chemical stability. that, due to metallic bonding, is malleable, ductile, and a good conductor. Mixed Groups 2. Groups 1 and 1. Groups 13 through 16 include metals, non- 2 are the alkali metals, and metalloids. and alkaline earth metals, which have some similar and some contrasting properties. 3. The iron triad, the coinage met- als, and the ele- ments in Group 12 are examples of transition elements. 4. The and have atomic 2. Allotropes are forms of the same element numbers 58 through 71 and 90 through having different molecular structures. 103, respectively. 3. The properties of three forms of carbon- graphite, diamond, and buckminster- Nonmetals fullerene-depend upon the differences in 1. Nonmetals can be brittle and dull. They are their crystal structures. also poor conductors of electricity. 4. All synthetic elements are short-lived. Except 2. As a typical nonmetal, hydrogen is a gas for technetium-43 and promethium-61, they that forms compounds by sharing electrons have atomic numbers greater than 92 and with other nonmetals and by forming ionic are referred to as transuranium elements. bonds with metals. These elements are found toward the bot- tom of the periodic table. 3. All the , Group 17, have seven outer electrons and form covalent and ionic compounds, but each has some I:ElI!D Use the Foldable that you made at the begin- properties that are unlike each of the others ning of this chapter to help you review elements and their properties. in the group.

\ Sc:ienc: II gpscience.comli nteractive_tutor CHAPTER STUDY GUIDE 595 • 9. Which set of elements makes up the most reactive group of all metals? allotrope p.585 radioactive element p.572 A) iron triad diatomic molecule p.579 salt p.580 B) ductile p.570 semiconductor p.585 C) alkali metals malleable p.570 sublimation p.581 metal p.570 transition element p.574 0) alkaline earth metals metallic bonding p.571 transuranium element 10. Which element is the most reactive of all metalloid p.584 p.589 nonmetals? nonmetal p.578 A) fluorine C) hydrogen B) uranium 0) oxygen Complete each sentence with the correct vocabll- lary word(s). 11. Which element is always found in nature combined with other elements? 1. The are located to the left of the A) copper C) magnesium stair-step line on the periodic table. B) gold 0) silver 2. Different structural forms of the same 12. Which elements are least reactive? element are called _ A) metals () noble gases 3. Positively charged ions are surrounded by B) halogens 0) actinides freely moving electrons in _ 13. What element is formed when neptunium 4. A(n) is a molecule comprised of disintegrates? two atoms. A) ytterbium () americium 5. The are in Groups 3 through 12 B) promethium 0) plutonium on the periodic table.

ec mg Concepts 14. Copy and complete the concept map using the following: trallsitioll elements, hydrogen, Choose the word or phrase that best allswers the metals, illHer transition metals, noble gases. I]lIestioll. 6. When magnesium and fluorine react, what type of bond is formed? A) metallic C) covalent B) ionic 0) diatomic are are 7. What type of bond is found in a piece of Alkali metals and alkaline pure gold? earth metal oand A) metallic () covalent and B) ionic 0) diatomic Halogens except 8. Because electrons move freely in metals, which property describes metals? oand and A) brittle C) dull B) hard 0) conductors

'1 596 CHAPTER REVIEW Scienc gpscience.com/vocabulary_puzzlemaker • 22. Compare and Contrast Explain why aluminum is a metal and carbon is not. 15. Concept Map Copy and complete the concept 23. Explain What is metallic bonding? Explain map using the following: Nn, Fe, Actinides, how this affects conductivity. Hg, Bn, Alknli, nnd Inner tmnsition. 24. Describe the geometric shapes of the carbon allotropes.

Use the following table to answer question 25.

Gas Analysis Gas Volume % 16. Make and Use Tables Use the periodic table CO 6.8 to classify each of the following as a H) 47.3 or actinide: , CH 33.9 , cerium, , terbium, 4 and uranium. CO) 2.2 N) 6 17. Explain why mercury is rarely used in ther- Other 3.8 mometers that take body temperatures.

18. Explain The density of hydrogen is lower 25. Interpret Data When coke-oven gas is than air and can be used to fill balloons. burned in an industrial process, several Why is helium used instead of hydrogen? gases are produced in the reaction. If 19. Explain Copper is a good choice for use in 385 grams of coke-oven gas are con- electrical wiring. What type of elements sumed in this reaction, how many grams of CH (methane) are produced? would not work well for this purpose? 4 Why? 26. Use Percentage Chloroform has the chemical formula, CHCl , and a molec- 20. Explain why various silver compounds are 3 ular weight of 119.39 g. What is the used in photography. percentage of Cl (chlorine) present in 21. Describe Like selenium, chromium is poi- this compound? sonous but is needed in trace amounts in 27. Use Numbers Calculate the molecular your diet. How would you apply this weight of bromide (GaBr ). information in order to use vitamin and 3 mineral pills safely?

, Scienc gpscience.com!chapterJeview CHAPTER REVIEW 597 - ---- , - _. -, --

Record your allswers all the answer sheet Use the illustration below to answer questions 5 and 6, provided by your teacher or on a sheet ofpaper. Elements in the Human Body 1. Which of these elements is the main com- 70,------, ponent of steel, and the most widely used 60 of all metals? .. 50 A. iron Co cadmium 40 B. aluminum D. magnesium .... 30- 2. What term describes the Group 1 elements 20 lithium, sodium, and potassium? 10 o A. alkali metals ",<:- 0<:- ",<:- ",<:- ,;;/:>>1; B. radioactive elements """ " Co lanthanides Elements D. transition metals Use the illustration below to answer questions 3 and 4. s. Which of these is a property of the ele- 26 27 28 ments that make up 98 percent of the Fe Co Ni r human body? A. malleability ;: B. poor electrical and heat conductivity Co shiny appearance , D. ductility

6. In which of these phases does the element 3. What name is given to these three elements present in the highest percentage in the which are used in processes that create steel human body exist? and other metal mixtures? A. gas Co liquid A. halogens Co actinides B. solid D. plasma B. the coin metals D. the iron triad 4. To which major group do these elements 7. Which element is present in all organic belong? compounds? A. nonmetals Co transition elements A. silicon Co nitrogen B. noble gases D. alkali metals B. m.'ygen D. carbon

8. Which of these is NOT a property of transuranium elements? Eliminate Choices If you don't know the answer to a A. occur naturally multiple-choice question eliminate as many incorrect choices B. have greater than 92 protons as possible. Co are synthetic D. are unstable

598 STANDARDIZED TEST PRACTICE Record YOl/r al/slI'ers 01/ the allSlI'er sheet Record YOl/r nllSIvers on the answer sheet provided by YOl/r teacher or 01/ a sheet ofpaper. provided by your teacher or on a sheet ofpaper. 9. Define the general properties of metals 16. Recent Federal Drug Administration state- which make them useful and versatile ments advise limiting consumption of tuna materials. and salmon. Which transitional element is the source of the problem? Explain why 10. Use the electron configuration of the ele- this element poses a potential risk. ments sodium and potassium to explain why these elements do not occur in nature 17. Use the properties of metallic bonds to in elemental form. explain why metal hammered into sheets Use the illustrations below to answer questions 11 does not break, as well as why metals con- and 12. duct electricity. 18. Based on its electron configuration and position in the periodic table, explain why fluorine is the most chemically active of all elements. Use the illustration below to answer question 19.

11. Define the term allotrope, and identify these allotropes of carbon. 12. Compare the structures of these carbon allotropes and relate the structures to the properties of these materials. 19. Identify the gas which enables this blimp 13. Describe some unique properties of to remain suspended in the atmosphere. hydrogen. Why would it be dangerous to use hydro- 14. Identify and describe the uses of some of gen for this purpose? the halogens obtained from seawater. 20. Explain the importance of organisms that 15. Compare the two types of bonds which convert nitrogen from its diatomic form nonmetals can form. into other compounds.

\ Selene 9psci ence.com/standard ized_test STANDARDIZED TEST PRACTICE 599

.. section I • Stability in Bonding

Ww You'll Learn W'v It's Important Review Vocabulary • Describe how a compound differs The millions of different kinds compound: substance formed from from its component elements. of matter around us are a result two or more elements in which the • Explain what a chemical formula of chemical bonds. exact combination and proportion of represents. elements is always the same • Explain that the electric forces between oppositely charged elec- New Vocabulary trons and protons are essential to • chemical formula forming compounds • chemical bond • State a reason why chemical bonding occurs.

Figure 1 The difference Combined Elements between the elemental copper Have you ever noticed the color ofthe Statue ofLiberty? Why metal and the copper compound is it green? Did the sculptor purposely choose green? Why wasn't formed on the Statue of Liberty white, or tan, or even some other color like purple chosen? Vvas is striking. it painted that way? No, the Statue of Liberty was not painted. The Statue of Liberty is made of the metal copper, which is an Elemental copper element. Pennies, too, are made of copper. Wait a minute, you say. Copper isn't green-it's ... well, copper colored. You are right. Uncombined, elemental copper is a bright, shiny copper color. So again the question arises: Why is the Statue of Liberty green?

Compounds Some of the matter around you is in the form of uncombined elements such as copper, sulfur, and o},,)'gen. But, like many other sets of elements, these three elements unite chemically to form a compound when the conditions are right. The green coating on the Statue of Liberty and some old pen- nies is a result of this chemical change. One compound in this coating, seen in contrast with elemental copper in Figure 1, is a new compound called copper sulfate. Copper sulfate isn't shiny and copper colored like elemental copper. Nor is it a pale-yellow solid like sulfur or a colorless, odorless gas like o>..)'gen. It has its Surface coated with a copper own unique properties. compound

602 CHAPTER 20 Chemical Bonds New Properties One interesting observation you will make is that the com- pound formed when ele- II""'" ments combine often has properties that aren't any- thing Ilke those of the indi- vidual elements. Sodium chloride, for example, shown in Figure 2, is a compound made from the ". elements sodium and chlo- Na + :(1" rine. Sodium is a shiny, soft, silvery metal that Sodium + Chlorine Sodium chloride reacts violently with water. Chlorine is a poisonous greenish-yellow gas. Would you have Figure 2 Sodium is a soft, guessed that these elements combine to make ordinary table salt? silvery metal that combines with chlorine, a greenish-yellow gas Formulas represented here as only one atom, to form sodium chloride, which is a The chemical symbols Na and CI represent the elements white crystalline solid. sodium and chlorine. When written as NaCl, the symbols make up Describe how the properties of a formula, or chemical shorthand, for the compound sodium table salt are different from those chloride. A chemical formula tells what elements a compound ofsodium and eMorine. contains and the exact number of the atoms of each element in a unit of that compound. The compound that you ate probably most familiar with is Table 1 Some Familiar Compounds H20, more commonly known as water. This formula contains the symbols H for the Familiar Name Chemical Name Formula element hydrogen and ° for the element oxygen. Notice the subscript number 2 Sand Silicon dioxide Si02 written after the H for hydrogen. Subscript means "written below." A subscript written Milk of magnesia Magnesium hydroxide Mg(OH)2 after a symbol tells how many atoms ofthat element are in a unit of the compound. If a Cane sugar Sucrose CnH22011 symbol has no subscript, the unit contains lime Calcium oxide CaO .only one atom of that element. A unit of

H20 contains two hydrogen atoms and one Vinegar Acetic acid CHJCOOH OA!'gen atom. Look at the formulas for each com- Laughing gas Dinitrogen oxide N20 pound listed in Table 1. What elements combine to form each compound? How Grain alcohol Ethanol C2HsOH many atoms of each element are required to form each of the compounds? Battery acid Sulfuric acid H2SO4

., Describe whata ehemi- Stomach acid Hydrochloric acid HCI . cal formula tells you.

SECTION 1 Stability in Bonding 603 1 2 13 14 15 16 17 18

li Be BCN 0 F Ne

2 •• • •• • .. •• .. •• • •• ••••• •• • •• ••••••• • •• • ••••• • • • • • • •••• •• •• ••

Na Mg AI Si P S CI Ar • •• • 3 •• •• •• •• .. •• •• •• •• •• •• •• •• •• •• •• •• ••••••• ••• •• •• • • • • • ••• • • • • • •••• • • • • • • • •• •••• •• ••• • •• •• •• ••••• •••• • •• •• ••• •• •• ••

Figure 3 Note in this simplified Atomic Stability representation of electron distribu- \ilJhy do atoms form compounds? The electric forces between tion, that the number of electrons oppositely charged electrons and protons hold atoms and mole- in each group's outer level cules together, and thus are the forces that cause compounds to increases across the table, until the form. The periodic table on the inside back cover of your book noble gases in Group 18 have a lists the known elements, most ofwhich can combine with other complete outer energy level. elements. However, the six noble gases in Group 18 seldom form Analyze Which ofthe groups pictured could form a diatomic compounds. Why is this so? Atoms of noble gases are unusually compound with the elements in stable. Compounds of these atoms rarely form because they are Group 21 almost always less stable than the original atoms.

The Unique Noble Gases To understand the stability of the noble gases, it is helpful to look at electron dot diagrams. Electron dot diagrams show only the electrons in the outer Figure 4 Electron dot diagrams energy level of an atom. They contain the for of noble gases show that they all the element surrounded by dots representing its outer electrons. have a stable, filled outer energy How do you know how many dots to make? For Groups 1 and 2 level. and 13 through 18, you can use a periodic table or the portion e _ of it shown in Figure 3. Look at the outer ring of each of the ele- e e ments. Group I has one outer electron. Group 2 has two. Group 13 has three, Group 14, four, and so on to Group 18, the noble He :Kr:e e gases, which have eight.

e e e e Chemical Stability An atom is chemically stable when its outer energy level is complete. Recall that the outer energy lev- :Ne:_ e :Xe:_ e els of helium and hydrogen are stable with two electrons. The e _ _ e outer energy levels of all the other elements are stable when they contain eight electrons. The noble gases are stable because they each have a complete outer energy level. Figure 4 shows electron _ e :Ar:e e :Rn: dot diagrams of some of the noble gases. Notice that eight dots surround Kr, Ne, Xe, Ar, and Rn, and two dots surround He.

604 CHAPTER 20 Chemical Bonds