Modern inorganic chemistry

344 GROUP VII: THE HALOGENS

iodide, for example aluminium trifluoride, A1F 3 , is ionic but the

remaining aluminium halides are all essentially covalent.

When an element has more than one oxidation state the lower

halides tend to be ionic whilst the higher ones are covalent—the

anhydrous chlorides of lead are a good example, for whilst lead(II)

chloride, PbCl 2 , is a white non-volatile solid, soluble in water

without hydrolysis, lead(IV) chloride, PbCl 4 , is a liquid at room

temperature (p. 200) and is immediately hydrolysed. This change of

bonding with oxidation state follows from the rules given on p. 49.

The solid anhydrous halides of some of the transition metals are

often intermediate in character between ionic and covalent; their

structures are complicated by (a) the tendency of the central metal

ion to coordinate the halide ions around it, to form an essentially

covalent complex, (b) the tendency of halide ions to bridge, or link,

two metal ions, again tending to covalency (cf. aluminium chloride,

p. 153 and iron(III) chloride, p. 394).

SOLUBILITY

Many ionic halides dissolve in water to give hydrated ions. The

solubility of a given halide depends on several factors, and generali-

sations are difficult. Ionic fluorides, however, often differ from other

halides in solubility. For example, calcium fluoride is insoluble but

the other halides of calcium are highly soluble; silver fluoride, AgF,

is very soluble but the other silver halides are insoluble.

Covalent halides

These are formed by less electropositive elements. They are charac-
terised by the existence of discrete molecules which exist even in the
solid state. They have generally lower melting and boiling points
than the ionic halides, are more volatile and dissolve in non-polar
solvents.
The melting and boiling points of a series of similar covalent
halides of a given element are found to increase from the fluoride to
the iodide, i.e. as the molecular weight of the halide increases. Thus,
the trihalides of phosphorus have melting points PF 3 = 121.5 K.
PC1 3 = 161.2 K, PBra = 233 K, PI 3 = 334 K.
Most covalent halides are hydrolysed by water (carbon tetra-
chloride being a notable exception, p. 195) to give acidic solutions,
by either method (a) (example FeCl 3 ) or method (b) (example BC1 3 )'