Chemistry, Third edition

IONIC AND COVALENT COMPOUNDS – TWO EXTREMES

Polarization in ionic compounds

A small, positively charged ion (a cation) in an ionic compound can attract the elec-

trons of a neighbouring negatively charged ion (an anion) towards it and distort the

anion. When this happens, the anion is said to be polarized. This distortion can be

better represented by using the electron density model of an ionic compound, rather

than using Lewis symbols. The outer electron density contours of a purely ionic

bond, and an ionic bond which is polarized, are shown in Fig. 4.7.

Notice that, in the polarized case, more electron density is concentrated between

the ions. A region of high electron density between atoms is characteristic of

covalent compounds, so that the polarized compound in Fig. 4.7(b), although still

mainly ionic, has some covalent character. When is significant polarization likely to

occur? We can answer this question by applying the following rules.

The polarization of an ionic bond (i.e. its covalent character) is high if:

1.The cation is (a) small and (b) carries a high positive charge.

2.The anion is (a) large and (b) carries a high negative charge.

Compounds in which both conditions apply will have a great deal of covalent

character. An example of a strongly polarizing cation is Al3+, which is small and car-

ries a large positive charge. The anion Iis large; the degree of polarization in AlI 3 is

so large, that it is considered to be a covalent compound. However, the compound

AlF 3 has far more ionic character, because Fis a much smaller anion and is not

polarized to the same degree as I.

59

Fig. 4.7Polarization: (a)

purely ionic; (b) polarized.

Cation Radius/pm Anion Radius/pm

Li 160 F 136

Na 95 Cl 181

K 133 S^2  184

Mg^2  165

Al^3  153

(i) In the series of ionic chlorides: LiCl, NaCl

and KCl, which compound would have

the most covalent character?

(ii) From the selection of anions and cations

above, which compound would you

expect to be the most ionic?

(iii) Which compound would you expect to

have the most covalent character?

Exercise 4J

Ionic and covalent bonding: a summary

The overall picture is summed up in Fig. 4.8. Again, the outer contours of electron

density maps have been used instead of Lewis symbols.

Fig. 4.8An overview of bonding types.

Polarization

The sizes of some cations and anions are

listed below:

Polarized ionic

e.g.Cal 2

Polar covalent

e.g.Hl

Purely covalent

e.g.Cl 2

Clearly ionic

e.g.KF