The Foundations of Chemistry

Almost all bonds have both ionic and covalent character. By experimental means, a
given type of bond can usually be identified as being “closer” to one or the other extreme
type. We find it useful and convenient to use the labels for the major classes of bonds to
describe simple substances, keeping in mind that they represent ranges of behavior.
Above all, we must recognize that any classification of a compound that we might
suggest based on electronic properties mustbe consistent with the physical properties of
ionic and covalent substances described at the beginning of the chapter. For instance, HCl
has a rather large electronegativity difference (0.9), and its aqueous solutions conduct elec-
tricity. But we know that we cannot view it as an ionic compound because it is a gas, and
not a solid, at room temperature. Liquid HCl is a nonconductor.
Let us point out another aspect of the classification of compounds as ionic or covalent.
Not all ions consist of single charged atoms. Many are small groups of atoms that are
covalently bonded together, yet they still have excess positive or negative charge. Exam-
ples of such polyatomic ionsare ammonium ion, NH 4 , sulfate ion, SO 42 , and nitrate ion,
NO 3 . A compound such as potassium sulfate, K 2 SO 4 , contains potassium ions, K, and
sulfate ions, SO 42 , in a 2:1 ratio. We should recognize that this compound contains both
covalent bonding (electron sharing withineach sulfate ion) and ionic bonding (electro-
static attractions betweenpotassium and sulfate ions). We classify this compound as ionic,
however, because it is a high-melting solid (mp 1069°C), it conducts electricity both in
molten form and in aqueous solution, and it displays other properties that we generally
associate with ionic compounds. Put another way, covalent bonding holds each sulfate ion
together, but the forces that hold the entiresubstance together are ionic.
In summary, we can describe chemical bonding as a continuum that may be repre-
sented as

(EN) for the

bonding atoms zero 88nintermediate 88nlarge

Bonding types nonpolar covalent88npolar covalent88nionic

HCl ionizesin aqueous solution. We

will study more about this behavior in

Chapter 10.

Key Term s299

Ionic character increases

Covalent character increases

Key Terms

Anion A negatively charged ion; that is, an ion in which the atom
or group of atoms has more electrons than protons.
Binary compound A compound consisting of two elements; may
be ionic or covalent.
Bonding pair A pair of electrons involved in a covalent bond.
Also called shared pair.
Cation A positively charged ion; that is, an ion in which the atom
or group of atoms has fewer electrons than protons.

Chemical bonds Attractive forces that hold atoms together in

elements and compounds.

Covalent bond A chemical bond formed by the sharing of one

or more electron pairs between two atoms.

Covalent compound A compound containing predominantly

covalent bonds.

Debye The unit used to express dipole moments.