The Foundations of Chemistry

As our previous discussions of ionization energy, electronegativity, and electron affinity
would suggest, ionic bonding can occur easily when elements that have low ionization
energies (metals) react with elements having high electronegativities and very negative
electron affinities (nonmetals). Many metals are easily oxidized—that is, they lose elec-
trons to form cations; and many nonmetals are readily reduced—that is, they gain electrons
to form anions.

When the electronegativity difference, (EN), between two elements is large, as

between a metal and a nonmetal, the elements are likely to form a compound by

ionic bonding (transfer of electrons).

Let us describe some combinations of metals with nonmetals to form ionic compounds.

Group IA Metals and Group VIIA Nonmetals

Consider the reaction of sodium (a Group IA metal) with chlorine (a Group VIIA
nonmetal). Sodium is a soft silvery metal (mp 98°C), and chlorine is a yellowish-green
corrosive gas at room temperature. Both sodium and chlorine react with water, sodium
vigorously. By contrast, sodium chloride is a white solid (mp 801°C) that dissolves in water
with no reaction and with the absorption of just a little heat. We can represent the reac-
tion for its formation as

2Na(s)Cl 2 (g) 88n 2NaCl(s)

sodium chlorine sodium chloride

We can understand this reaction better by showing electron configurations for all species.
We represent chlorine as individual atoms rather than molecules, for simplicity.

11 Na [Ne] __

h Na [Ne] 1 e lost

3 s

88n

17 Cl [Ne] __

hg __hg __hg__h Cl [Ne] __hg __hg __hg __hg 1 egained

3 s 3 p 3 s 3 p

Freshly cut sodium has a metallic

luster. A little while after being cut,

the sodium metal surface turns white

as it reacts with the air.

7-2 Formation of Ionic Compound s273

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Some ionic compounds. Clockwise

from front right: sodium chloride

(NaCl, white); copper(II) sulfate

pentahydrate (CuSO 4 5H 2 O, blue);

nickel(II) chloride hexahydrate

(NiCl 2 6H 2 O, green); potassium

dichromate (K 2 Cr 2 O 7 , orange); and

cobalt(II) chloride hexahydrate

(CoCl 2 6H 2 O, red). One mole of

each substance is shown.