Modern inorganic chemistry

GROUP VII: THE HALOGENS 343

Anhydrous halides, however, are obtained when the metal is

heated with the dry hydrogen halide or the halogen. In the case of

elements with more than one oxidation state, the hydrogen halide

produces a lower halide and the halogen a higher halide, for example

Sn + 2HC1 -» SnCl 2 + H 2 T

Sn + 2C1 2 -» SnCl 4

The higher iodides, however, tend to be unstable and decomposition

occurs to the lower iodide (PI 5 -» PI 3 ). Anhydrous chlorides and

bromides of some metals may also be prepared by the action of

acetyl (ethanoyl) halide on the hydrated ethanoate (acetate) in

benzene, for example cobalt(II) and nickel(II) chlorides:

Co(CH 3 COO) 2 + 2CH 3 COC1 + 2H 2 O -> CoCl 2 i + 4CH 3 COOH

Sulphur dichloride oxide (thionyl chloride) on the hydrated chloride

can also be used to produce the anhydrous chloride in certain cases,

for example copper(II) chloride and chromium(III) chloride:

CrCl 3. 6H 2 O + 6SOC1 2 -> 6SO 2 t + 12HC1T + CrCl 3

Halides of non-metals are usually prepared by the direct com-

bination of the elements. If the element exhibits more than one

oxidation state, excess of the halogen favours the formation of the

higher halide whilst excess of the element favours the formation of

the lower halide (e.g. PC1 5 and PC1 3 ).

Ionic (salt-like) halides

These are halides formed by highly electropositive elements (for
example those of Groups I and II, except for beryllium and lithium).
They have ionic lattices, are non-volatile solids, and conduct when
molten; they are usually soluble in polar solvents in which they
produce conducting solutions, indicating the presence of ions.
The change from ionic to covalent bonding is gradual in a given
group or period; for a given halogen, as the size of the metal ion
decreases and more especially as its charge increases, the degree of
covalency increases. Thus, for example, in the chlorides of the four
elements, potassium, calcium, scandium and titanium, i.e. KC1,
CaCl 2 , ScCl 3 and TiCl 4 , KC1 is essentially ionic, TiCl 4 is essentially
covalent.
When the several halides of a given element are considered,
changes in bond character are also found. The fluoride is generally
the most ionic with ionic character decreasing from fluoride to