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energy during a chemical reaction, and other reactions absorb en- covalent bond is formed by the sharing of a pair of electrons, with each
ergy. A chemical equation is a shorthand way of describing a chemi- atom contributing a single electron to the shared pair. Covalent bonds
cal reaction. An equation shows the substances that are changed, the formed when two pairs of electrons are shared are called double bonds.
reactants, on the left side, and the new substances produced, the prod- A triple bond is the sharing of three pairs of electrons.
ucts, on the right side. The electron-pulling ability of an atom in a bond is compared with
Chemical reactions involve valence electrons, the electrons in arbitrary values of electronegativity. A high electronegative value means
the outermost orbital of an atom. Atoms tend to lose or acquire elec- a greater attraction for bonding electrons. If the absolute difference in
trons to achieve the configuration of the noble gases with stable, electronegativity of two bonded atoms is 1.7 or more, one atom pulls the
filled outer orbitals. This tendency is generalized as the octet rule, bonding electron away, and an ionic bond results. If the difference is less
that atoms lose or gain electrons to acquire the noble gas structure of than 0.5, the electrons are equally shared in a covalent bond. Between
eight electrons in the outer orbital. Atoms form negative or positive 0.5 and 1.7, the electrons are shared unequally in a polar covalent bond.
ions in the process. A polar covalent bond results in electrons spending more time around
A chemical bond is an attractive force that holds atoms together the atom or atoms with the greater pulling ability, creating a negative
in a compound. Chemical bonds that are formed when atoms transfer pole at one end and a positive pole at the other. Such a molecule is called
electrons to become ions are ionic bonds. An ionic bond is an elec- a dipole, since it has two poles, or centers, of charge.
trostatic attraction between oppositely charged ions. Chemical bonds Compounds are named with systematic rules for ionic and covalent
formed when atoms share electrons are covalent bonds. compounds. Both ionic and covalent compounds that are made up of
Ionic bonds result in ionic compounds with a crystalline struc- only two different elements always end with an -ide suffix, but there are
ture. The energy released when an ionic compound is formed is called a few -ide names for compounds that have more than just two elements.
the heat of formation. It is the same amount of energy that is required The modern systematic system for naming variable-charge ions
to decompose the compound into its elements. A formula of a com- states the English name and gives the charge with Roman numerals
pound uses symbols to tell what elements are in a compound and in in parentheses. Ionic compounds are electrically neutral, and formulas
what proportions. Ions of representative elements have a single, fixed must show a balance of charge. The crossover technique is an easy way
charge, but many transition elements have variable charges. Electrons to write formulas that show a balance of charge.
are conserved when ionic compounds are formed, and the ionic com- Covalent compounds are molecules of two or more nonmetal
pound is electrically neutral. The formula shows this overall balance atoms held together by a covalent bond. The system for naming cova-
of charges. lent compounds uses Greek prefixes to identify the numbers of atoms,
Covalent compounds are molecular, composed of electrically since more than one compound can form from the same two elements
neutral groups of atoms bound together by covalent bonds. A single (CO and CO 2 , for example).

KEY TERMS APPLYING THE CONCEPTS

atom (p. 230) 1. Which of the following is not true of a compound? It is
chemical bond (p. 233) a. a pure substance.
chemical energy (p. 231) b. composed of combinations of atoms.
chemical equation (p. 231) c. held together by chemical bonds.
chemical reaction (p. 231) d. a substance that cannot be broken down into simpler units.
covalent bond (p. 236) 2. The smallest unit of an element that can exist alone or in
covalent compound (p. 237) combination with other elements is a (an)
a. nucleus.
double bond (p. 238)
b. atom.
electronegativity (p. 239)
c. molecule.
formula (p. 235)
d. proton.
heat of formation (p. 234)
3. The smallest particle of a compound or a gaseous element that
ionic bond (p. 234)
can exist and still retain the characteristic properties of that
ionic compounds (p. 235)
substance is a (an)
molecule (p. 230) a. molecule.
octet rule (p. 232) b. element.
polar covalent bond (p. 239) c. atom.
single bond (p. 238) d. electron.
triple bond (p. 238) 4. Which of the following is an example of a monatomic molecule?
valence (p. 244) a. O 2
valence electrons (p. 232) b. N 2
c. Ar
d. O 3

246 CHAPTER 9 Chemical Bonds 9-18