Hazardous Materials

4,3 CHEMICAL AND PHYSICAL CHANGES The constant co~position associated with a given substance is maintained by internal linkages among its units. These linkages are called chemical bonds. When a particular process occurs _that makes or breaks these bonds, we say that a chemical change, or a chemical reaction, has occurred. Combustion and corrosion are common examples of chemical changes associated with some hazardous materials.

Let's briefly consider the nature of the combustion process. When something burns, it combines with oxygen. The resulting products of combustion are compounds containing oxygen, called oxides. For instance, many commercial gasolines are mixtures of several substances, including one called octane. When octane burns completely, it becomes car- bon dioxide gas and water vapor. Carbon dioxide is an oxide of carbon, as its name implies, whereas water is an oxide of hydrogen. Carbon dioxide and water vapor are unlike octane or other gasoline components. They have different properties and different compositions. This conversion of octane to carbon dioxide and water is typical of a chem- ical change.

By contrast, substances can undergo changes during which their compositions remain the same. Such changes are called physical changes. Let's consider octane again. Some of its physical and chemical changes are illustrated in Figure 4.2. When exposed to the ambi- ent environment, liquid octane evaporates, but its chemical composition remains unchanged. Such alterations in the physical state of a substance, such as from a liquid to a vapor, are considered physical changes. Other examples of physical changes are melting, freezing, boiling, crushing, and pulverizing.

The types of behavior that a substance exhibits when undergoing chemical changes are called its chemical properties. The characteristics that do not involve changes in the chemical identity of a substance are called its physical properties. All substances can be distinguished from one another by these properties, in much the same way as certain fea- tures-fingerprints or DNA, for example-distinguish one human being from another. The study of hazardous materials is concerned to a great extent with learning the chemi- cal and physical properties of appropriate substances, some examples of which are listed in Table 4.3.

r

GASOLINE

FIGURE 4 .2 Examples of physical and chemical changes in the compon~nts of gasoline. On the left, the com- ponents evaporate ; that is, they change their physical state fr?m the liquid to th7 vapor: This phenomenon con- stitutes a physical change . On the right, a spill of gasoline 1gn1tes ~nd burns, d~nng which the components become carbon dioxide and water vapor. This phenomenon constitutes a chem,cal change.

chemical bon d The force by which the atoms of one element become attached to, or associated with, other atoms in a compound

ch emical chan ge (che mi ca l reac- tion) Any modifica- tion or transformation that results in an altera- tion of the chemical identity of a substance

physical ch ange • A modification or trans- formation that does not result in an altera- tion of the chemical identity of a substance

chem ica l prop erty • Any type of behavior that a substance exhib- its when it undergoes a chemical change

physi cal property • Any phenomenon that a substance exhibits when it undergoes a physical change

Chapter 4 Chemical Forms of Matter 111

llli.:._

atom The smallest particle of an element that can be identified with that element

electron • An atomic particle that has an electric charge of -1

proton A particle in the nucleus of an atom that has an electric charge of +1

neutron A particle in the nucleus of an atom that has no electric charge

atomic nucleus • Th e region at the center of an atom occupied by protons and neutrons

at omic orbital • The region in the space around an atomic nucleus in which electrons are most likely to be found

TABLE 4.3 Characteristics of Some Substances

SUBSTANCE PHYSICAL PROPERTIES CHEMICAL PROPERTIES

Oxygen, an element Odorless, colorless gas; does not Combines readily with many

conduct heat or electricity; density = elements (a chemical reaction 1.43 g/L; becomes liquid at -297'F called oxidation)

(-183'C)

Phosphorus, White or red solid; does not conduct Readily combines with oxygen,

an element heat or electricity; density = 1.82 g/ chlorine, and fluorine; white form ml (white) and 2.34 g/mL (red) spontaneously ignites in dry air

Carbon dioxide, Odorless and colorless gas; solidifies Does not burn; reacts with water-

a compound at -83'F (-67'C) under pressure, soluble metal compounds, forming

forming dry ice; soluble in water metallic carbonates

under pressure

Hydrogen chloride, Strong-smelling, colorless gas; Reacts with many minerals, formin,;-

a compound density = 1.20 g/mL; soluble in water-soluble products; reacts with water, forming hydrochloric acid ammonia, forming ammonium

chloride

4 .4 SOME BASIC FEATURES OF ATOMS If a small piece of an element, say aluminum, could be hypothetically divided and subdivided into smaller and smaller pieces until subdivision was no longer possible, the result would be one particle of aluminum. This smallest particle of the element that is still representative of the element is called an atom, from the Greek word atomos, meaning " indivisible."

Although an atom is infinitesimally small, it is also composed of even smaller par ri· des known as electrons, protons, and neutrons. Electrons are negatively charged particles that are responsible for the chemical reactivity of a given element. Protons are positively charged particles, and neutrons are neutral particles . Electrons and protons bear the same magnitude of charge but are of opposite signs. For con venience, the electron has a charge of -1, the proton of +1, and the neutron of 0.

Protons are relatively heavy particles; they are 1836 times more massive than elec· trons . Neutrons are slightly more massive than protons. The fundamental characteristics of electrons, protons, and neutrons are summarized in Table 4.4.

The protons and neutrons of an atom reside in a central area called the atomic nucleus. Electrons reside primarily in designated regions of space surrounding the nucleus, called atomic orbitals. There are several types of atomic orbitals; some are close to the nucleus, whereas others are relativel y remote from it. Scientists ha ve learned

TABLE 4.4 Some Basic Atomic Particles

PARTICLE PROTON ELECTRON NEUTRON

Symbol p e n

Relative charge +1 -1 0

Relative mass 1 About 0' 1

aThe mass of an electron is 1/1836 the mass of the proton.

112 Chapter 4 Chemical Forms of Matter

rhal only a prescribed numbe r of electrons reside in a given type of atomic orbital. Two elecrrons a re alw ays close to the nucleus, in an atom 's innermost atomic orbital (with rhe exception of a hydrogen atom,_ which possesses only one electron) . Most atoms have addirion al electrons m the atomic orbitals that are located some distance from the nucleus .

The number of protons in an atom is called the atomic number. The atomic number is often used in the study of chemistry to determine the number of electrons possessed by a neurral atom of an element. An atom of hydrogen has one electron, helium has two, lithium has three, and so_ f?rth. Carbon is an element composed only of carbon atoms, and all carbon atoms ex?ibit ne~rly the same physical and chemical properties. Some car- bon aroms may have slightly different masses due to different numbers of neutrons in rheir nuclei, but they act the same when they undergo chemical changes. Similarly, oxygen is an element composed of oxygen atoms, and all oxygen atoms possess nearly the same properties. But carbon and oxygen atoms are not alike, because their atoms possess differ- enr numbers of electrons, protons, and neutrons.

Although neutra l atoms of the same element have an identical number of protons and electrons, they may differ by the number of neutrons in their nuclei. Atoms of the same element having different numbers of neutrons are called the isotopes of that element. We note later, in Chapter 16, that some isotopes are radioactive. When the isotopes of an ele- ment are not radioactive, they are said to be stable. The elements exist in nature as mix- tures of their stable isotopes. The majority have two or more. Forty-two (42) elements have no stable isotopes, and 18 have only one.

Over the past century, scientists have determined with considerable accuracy the absolute masses of the stable atoms of the known elements. Immediately apparent is the fact that the atomic masses are extremely small numbers. Because it is inconvenient to work with them, scientists selected a specific carbon isotope called carbon-12 as the atom whose mass now serves as the reference standard for assigning atomic masses to all the other stable isotopes. Each atom of carbon-12 has six electrons, six protons, and six neu- trons. It is assigned a mass of exactly 12.

Scientists use modern analytical instruments to determine the natural abundance of an element's stable isotopes in a sample of a terrestrial material. Then, they can calcu- lare their weighted average in the sample. A weighted average is determined by multi- plying the abundance of each stable isotope by its atomic mass and summing the products . The weighted average mass of the stable isotopes of an element in a given source is called the element's atomic weight. Because atomic weights are relative param- eters, they are unitless numbers. The atomic weights of the elements are numbers that tell us how the masses of their stable atoms from a given source compare with the mass of the carbon-12 atom .

When the natural abundances of an element's stable isotopes do not vary among the many sources from which they are obtained, the atomic weight of the element is calcu- lated as a single number. However, when the abundances of the stable isotopes of an ele- ment vary with the locations around the world from which the element is obtained, the atomic weight of the element is dependent on its source. When an element's isotopic abun- dance varies from sample to sample, its atomic weight also varies, thereby providing a range of values. This latter situation is especially true for hydrogen, lithium, boron, car- bon, nitrogen, oxygen, silicon, sulfur, chlorine, and thallium. The range of their atomic Weights is often written as two bracketed numbers separated by a semicolon. For exam- ple, the atomic weight of hydrogen is written as [1.00784; 1.00811], meaning that hydro- gen in a given source has an atomic weight that is a number equal to or greater than 1.00784 and equal to or less than 1.00811.

The atomic numbers and atomic weights of all the elements are listed in Appendix B at the back of this text.

atomic number The number of protons in an atomic nucleus; the number of electrons in an electrically neutral atom

isotope Any of a group of nuclei having the same number of protons but different numbers of neutrons

atomic we ight An abundance-weighted sum of the atomic masses of an element's naturally occurring isotopes from a specified source

Chapter 4 Chemical Forms of Matter 113

--

•1·1@i·liiiGfilll The analys is of multiple samples of stronti um collected from sources arou nd the world shows that the elern,nt has four stable isotopes: strontium -84, strontium-86, strontium-87, and strontium-88 - Their isotopic com P<>sition Is essentially independent of the sources from which the strontium is collected, and 15 noted below:

periodic law The observation that the chemical properties of the elements are peri- odic functions of their atomic numbers

periodic table A display of the known elements into periods and families, arranged by increasing atomic number so that ele- ments with similar chemical properties are located in the same column (or family)

family of e leme nts The group of elements listed in a vertical column of the periodic table

tsoto~ Atomic mass Natural abundance (%} 0.50

Strontium-84 Strontium-86 Strontium-87 Strontium-88

83 .9 134 85.9094 86.9089 87 .9056

9.90 7.00

82 .60

Show that the atomic wei ght of strontium in these mult iple samples is 87 ,62 -

Solution: The word percentage mea ns parts per hundred (Section 2.4); hence, 0.50o/o, 9.9o% , 7-00% , ano 82 .60 % equal 0.0050, 0.0990, 0.0700, and 0.8260, respectively. Because the atomic weight of an element_~ the wei ghted average of t he atom ic masses of its stable isotopes In a given sample, the atomic weight of strontium , calculated to be 87 .62 , as follows:

83.9 134 X 0.0050 = 0.4196 85.9094 X 0.0990 = 8.5050 86.9089 X 0.0700 = 6.0836 87 .90 56 X 0. 8260 = Zb§_lQQ

87 .62

4 .5 THE PERIODIC CLASSIFICATION OF THE ELEMENTS During the last half of the nineteenth century, several scientists first noted that the chemi- cal properries of any given element were similar to the chemical properries of certain other elements . For example, they noted that sodium metal reacts explosively with water and burns spontaneously in the air. When these two chemical properties of sodium were com- pared with the properties of other elements, they found that potassium also explodes on contact with water and burns spontaneously in air. These scientists summarized this observation in the periodic law: The properties of the elements vary periodically with their atomic numbers. The term periodic reflects this repetition of chemical properties.

Suppose we list each element in a square and then arrange the squares by order of increasing atomic number. This means that the total number of electrons possessed by each element increases in this arrangement, one at a time, as we move from one square to the next. Then, let's further arrange them into columns of elements that possess similar properties. Of course, we would need to know a great deal of chemistry to accomplish this feat. A similar exercise was first performed more than 130 years ago, when many el, ments known today had not yet been discovered.

Such an arrangement of the chemical elements into a chart designed to represent the periodic law is called a periodic table. Although a number of versions exist, the periodK table shown in Figure 4.3 provides significant information for emergency responders. In this version, the elements are grouped into columns numbered lA through 8A and 1B through 8B. In another common version, the columns are numbered consecutively across the tabk from 1 to 18. To avoid confusion, we shall use the first version exclusively.

!he elements positioned _withi_n the same column of the periodic table are called :i family of elements. Each family 1s 1dent1fied by a number and a capital letter at the rap the column, such as lA and 2A. Thus, for example, helium, neon, argon, krypton , xenoll, and radon belong to the same family, identified by 8A.

114 Chapter 4 Chemical Forms of Matter

IA Periodic Table of the Elements hllo //chemistry abo ut com <Cl 2012 Todd Helmenstine About Chemistry

8A

r.1>i

Lanthanides

Actinides 23203808 23!03588 2la02ag1 im 1 -tnoru.,,, ~""' U111eqr, N~-'ll.ti,

Alka li Alkaline Basic Metals Eortt, l.letal

FIGURE 4.3 A modern version of the periodic table of the elements. [Courtesy of Todd Helmenstine, About Chemistry (2010).]

Elements in the same row of a periodic table are said to belong to the same period. The periods are numbered on the far left of the table from 1 to 7. There is one period of 2 ele- ments, two periods of 8 elements, two of 18 elements, and two more periods of 32 elements.

The periodic table of the elements is one of the most powerful icons in science: a single table that consolidates an array of valuable information. Some version of the table hangs on the wall of nearly every chemistry laboratory throughout the world. Simply by glancing ar it, we can observe immediately the atomic number of any element. We can also readily distinguish among those elements that are metals, nonmetals, and metalloids. In Figure 4.3, the salmon-colored squares contain the symbols of the elements that are metalloids. They separate the metals from the nonmetals. Generally, the metals are the elements that fall to the left of the group, and the nonmetals are the elements that fall to the right of it.

The usefulness of a periodic table consists in the manner by which it displays the peri- odicity, or repetition, in the properties of the elements at regular intervals, In particular, when we observe the elements as members of the same family, we know that they possess similar chemical properties, Five families deserve special recognition in this regard. They are identified by unique names:

period • A horizontal row on the periodic table

alkali metal family The elements of Group 1 A on the periodic table: lithium, sodium, potassium, rubidium, cesium, and francium . 1 Group 1A is called the alkali metal family; its members are lithium, sodium, potas-

sium, rubidium, cesium, and francium. Each metal reacts with water, although lithium re- alkaline earth acts slowly, Each metal also ignites in air, especially when exposed to a moist atmosphere. family • The elements In Figure 4.3, the squares designating the alkali metals are colored hot pink. in Group 2A on

the periodic table: . 1

Group 2A is called the alkaline earth family; its members are beryllium, magne- beryllium, magnesium, s1 um, calcium, strontium, barium, and radium. These elements are also chemically reac- calcium, strontium,

tive, bur not nearly as reactive as the alkali metals. They cause water to decompose, but barium, and radium

Chapter 4 Chemical Forms of Matter 115

oxygen family The elements in Group 6A on the periodic table : oxygen, sulfur, sele- nium, tellurium, and polonium

halogen family The elements in Group 7A on the periodic table: fluo- rine, chlorine, bromine, iodine, and astatine

noble gas family • The elements in Group SA on the periodic table: helium, neon, argon, krypton, xenon, and radon

molecule • The smallest neutral unit of some elements and compounds, composed of at least two atoms

nonpolar substance • Any element or com- pound having a sym- metrical distribution of charges about its center

polar substance • Any element or compound having an asymmetrical distribution of charges about its center

the rate of decomposition is slow at ambient temperatures. They ignite in the air b f

. , Ut On! a ter they have been heated or exposed to an ignition source. In Figure 4.3, the sq Y designating the alkaline earth metals are colored medium blue. uares

Group 6A is called the oxygen family; its members are oxygen, sulfur, selen· tellurium, and polonium, each of which is a moderately active substance. In Figur/~• the squares designating the chalcogens are not uniquely colored as a group. · '

Group 7 A is called the halogen family; its members are fluorine, chlorine b 1 h f h

. h . , ro. mine, iodine, and astatine. These elements are nonmeta s, eac O w ic 1s especial] reactive. In Figure 4.3, the squares designating the halogens are col~red green. y

Group SA is called the noble gas family; its members are helium, argon, krypton xenon, and radon. Chemists originally thought that these gases were all inert to chemical combination and called them "inert gases." Although some of them, such as xenon, are now known to form compounds, the noble gases uniquely exist a~ a group of elements lacking the chemical reactivity observed for all other elements. In Figure 4.3, the squares designating the noble gases are colored teal.

- Because thallium compounds are highly toxic, t hey are sometimes commercially available in rodenticides, prod- ucts that kill rodents . Using the periodic table, answer the following questions:

(al What is the symbol for t hallium? (bl What are the symbols for the elements immediately adjacent to thallium on the periodic table? (cl Is thallium a metal, nonmetal, or metalloid? (dl Provide the symbols of all the elements in the family of which thallium is a member. (el Identify the group number of t he family of which thallium is a member.

Solution : (al Thallium is located in the sixth period. Its chemical symbol is Tl. (b) The elements immediately adjacent t o thallium on the periodic table are mercury and lead, whose symbol

are Hg and Pb, respectively. (c) Thallium is a metal because it is located to the left of the salmon-colored metalloids on the periodic table (dl Boron , aluminum , galli um, indium, and thallium are members of the same family of elements. (el Thallium is a mem ber of the fam ily of elements denoted as 3A.

4 .6 MOLECULES AND IONS Although the smallest representative particle of an element is the atom, not all uncom· bined elements exist as single atoms. In fact, only six elements actually exist as single atoms. They are the noble gases. They are said to be "monatomic."

Other gases or liquids at room conditions consist of units containing pairs of li ke atoms. They are called molecules. For example, hydrogen, oxygen, nitrogen, and chl ori ne are gaseous ~lements as we generally encounter them, each of which is composed

01 3

molecule havmg two atoms. These molecules are said to be "diatomic " and are symbol· ized by the formulas H2, 02, N2, and Cl2, respectively. Their structures are illustrated

10

Figure 4.4. Because their charges are symmetrical about their molecular centers, they are called nonpolar substances.

By contrast, the charges in substances like hydrogen fluoride and w ater a re uonsrm· metrical about t?e centers of their molecules. These compounds are ca lled polar substances. a~d a~e s~mbohzed by _the form~as HF and H20, respectivel y. Their structures are pr: v1ded m ~igure 4.5. Their nature 1s due to the congregation of positive a nd nega ti ve charg on opposite ends of the molecules.

Chapter 4 Chemical Forms of Matter

TABLE 4.5 Lewis Symbols of Some Representative Elements

FAMILY 1A 2A 3A 4A SA 6A 7A Li· ·Be· ·B· ·C· ·~· •o• :F'· H· Na · ·Mg· ·Al· ·S)· ·f ·s• :Cl· K· ·Ca· ·Ga· ·G~· ·J\s· ·$~· :~r· Rb· ·Sr· ·1~· :X-

Table 4.5 lists the Lewis symbols of some representative elements important to the srudy of hazardous materials. Note that a simple way exists for determining the number of valence electrons for any element of the A family: The number that identifies the A family on the periodic table is also the number of valence electrons possessed by elements in that family. For instance, the halogens are located in the family identified as Group 7 A, and we note from Table 4.5 that each halogen has seven valence electrons.

4.9 IONIC BONDING Electrons can be transferred from an atom of one element to an atom of another element, resulting in the formation of positive and negative ions. This phenomenon generally occurs between the atoms of metals and nonmetals. The ions that form are attracted to each other by virtue of their opposite charges. Chemists call this electrostatic force of attraction an ionic bond . The formation of the ionic bond is based on a fundamental law of nature by which forms of matter with like charges ( +/+ or -/-) repel each other, whereas forms of matter with unlike charges (+/-)attract.

Many atoms of the elements transfer or accept just the number of electrons that gives them eight electrons in their outermost atomic orbitals. By transferring only this number of electrons, these atoms attain the electronic stability of the noble gas nearest to them in atomic number.

For illustration, consider the ionic bonding in sodium fluoride. From Table 4.5 we learn that the Lewis symbols of sodium and fluorine are Na • and :f, respectively. When these two elements combine at the atomic level, an atom of sodium transfers its single electron to a fluorine atom. By transferring one electron, the sodium atom electronically resembles neon. By accepting it, the fluorine atom electronically resembles neon. The atoms become charged; that is, they become ions. The process can be written schemati- cally as follows :

Na · + :F • ----'> Na + : F :

The attraction between these oppositely charged ions constitutes the ionic bond that binds the two ions together.

4.10 COVALENT BONDING Electrons can also be shared by atoms of identical or different elements to form mole- cules of elements or compounds. This sharing of electrons is usually between nonmetal atoms. Atoms of the same nonmetal bond to one another and form molecules of the element; atoms of different nonmetals bond to one another and form molecules of a compound.

ionic bond The electrostatic force of attraction between oppositely charged ions

Chapter 4 Chemical Forms of Matter 119

covalent bond A shared pair of electrons between two atoms

Lewis structure A means of displaying the bonding between the atoms of a molecule by the use of dots or dashes for shared pairs of electrons

The a toms of nonmetals acquire th eir electronic stability by sharing electro . h . h mm manner t a t pernuts t em to resemble atoms of the noble gases nearest to them in _a

b F h · h f" atomic ~wn er. or t e atoms m t e IrSt and second periods other than hydrogen, helium 1 . h

mm, and berylliwn, this means sharing a total of eight electrons in their outermost a:o rt_· orbital. This includes the atom 's valence electrons plus those it shares with another at:ic Hydrogen atoms share only two electrons. One shared pair of electrons between any:· atoms is called a covalent bond. 0 . Let's observe how two atoms of hydrogen combine to form a hydrogen molecule. f-{. 1s the Lewis symbol for hydrogen. To achieve the electronic structure of helium, the near- est noble gas, one hydrogen a tom shares its only electron with the single electron from a second hydrogen atom. We represent this process as follows:

H · + H · -----> H: H

The pair of electrons shared between the two hydrogen atoms is a covalent bond. This manner of representing the hydrogen molecule (H:H) is called Lewis structure.

Two chlorine atoms combine to form a chlorine molecule. :Cl · is the Lewis symbol for chlorine. To achieve the electronic structure of argon, the neare;t noble gas, each chlorine atom shares its unpaired electron. This formation of the chlorine molecule from two clllo- rine atoms can be represented as follows:

: C l• + : C J· -----> : CJ : C J :

Let's consider next the combination of hydrogen and chlorine atoms. A hydrogen arom and a chlorine atom can share an electron pair to form a molecule of the substance called hydrogen chloride. The formation of a hydrogen chloride molecule from a hydrogen atom and a chlorine atom can be represented as follows :

H· + : CJ · -----> H : C J :

The hydrogen atom shares an electron pair, so its electronic structure resembles that of helium, whereas the chlorine atom shares an electron pair, so th a t its electronic structure resembles that of argon.

An atom can also form several covalent bonds with other atoms by simpl y sharing more than one pair of electrons. For instance, consider the formation of the methane mol- ecule. This molecule consists of one carbon atom a nd four hydrogen a toms. The Lewis symbols for carbon and hydrogen atoms are · C · and f-l ·, respectivel y. In the methane mol- ecule, the carbon atom shares each of its four electrons with the electrons from four hydrogen a toms, as represented here:

H · C · + 4 H · -----> H : C : H

H

The sharing of electrons in the methane molecule results in an electronic a rrangement like that of the neon atom for the carbon atom and an electronic arrangement like that of the helium atom for each of the four hydrogen atoms.

double bond A cova• lent bond composed of four electrons shared between two atoms

triple bond A cova- lent bond composed of six shared electrons between two atoms

Sometimes, two nonmetallic atoms share more than one pair of electrons . This beha v· ior is particularly characteristic of carbon atoms and results in the forma tion of multiple bonds. Two types of multiple bonds exist: double and triple bonds. A double bond con· sists of the sharing of two pairs of electrons (: :), whereas a triple bond consists of th e sharing of three pairs of electrons (:::). The name of the element is designated when rbe pairs of electrons are associated with atoms of the same element, as for exa mple, carbon· carbon double bond and carbon-carbon triple bond.

120 Chapter 4 Chemical Forms of Matter

H:H :Cl:CI: H:CI: Hydrogen Chlorine Hydrogen chloride

H H;¢:H Q::C::Q :C:::O:

H Metha ne Carbon dioxide Carbo n monoxide

The carbon dioxide molecule consists of one atom of carbon and two atoms of oxygen. h Lewis symbols for carbon and oxygen atoms are ·C· and ·O· respectively. The forma-

T e of a carbon dioxide molecule from one carbon at~m and't~o oxygen atoms can be non II

Se nred as fo ows: repre

·9 · + ·¢· + ·9· - o::c::o Notice the existence of two pairs of electrons on each side of the carbon atom in the car- bon dioxide molecule. Each of these shared pairs of electrons is a double bond. By sharing electrons in this fashion, the carbon atom and the two oxygen atoms all achieve the elec- tronic arrangement of neon.

The formation of the carbon monoxide molecule from a carbon atom and an oxygen atom can be represented as follows:

·C· + · O· - :c:::Q:

The three shared pairs of electrons between the carbon and oxygen atoms constitute a triple bond. Once again, the carbon and oxygen atoms achieve the electronic stability of the neon atom by sharing eight electrons between them. The Lewis structures of several other molecules are illustrated in Figure 4.6.

For the sake of simplicity, Lewis structures are usually written by drawing a long dash to represent the shared pair of electrons. The dots representing all other electrons are omitted. In this notation, the compounds previously noted can be represented as follows:

H I

H- H CI - Cl H-Cl H-C- H C=O O=C=O I H

Hydrogen Chlorine Hydrogen chloride Me thane Carbon monox ide Carbon dioxide

FIGURE 4 .6 The Lewis structures of some simple molecules.

SOLVED EXERCISE 4.3

At ambient conditions nitrogen trifluoride is a gas. It is used to manufacture item s like microchips and flat screen televisions. '

(a) What is the chemical formula of nitrogen trifluoride? (b) What is its Lewis structure?

Solution:

(a) From Table 4.9, we see that the Greek prefix tri- refers to three . The absence of a prefix fo r " nitro- gen" implies that the prefix mono- was dropped for si mplicity. This means that the nitrogen trifluoride

Chapter 4 Chemical Forms of Matter 121