Modern inorganic chemistry

342 GROUP VII; THE HALOGENS

CHLORATES(VII)

These can be prepared by electrolytic oxidation of chlorates(V) or

by neutralisation of the acid with metals. Many chlorates(VII) are

very soluble in water and indeed barium and magnesium chlorates-

(VII) form hydrates of such low vapour pressure that they can be

used as desiccants. The chlorate(VII) ion shows the least tendency

of any negative ion to behave as a ligand, i.e. to form complexes with

cations, and hence solutions of chlorates (VII) are used when it is

desired to avoid complex formation in solution.

The chlorate(VII) ion, C1O~, is isoelectronic with the sulphate(VI)

ion, SO^", and has a similar tetrahedral symmetry.

lodic(VII) acids

These are acids which can be regarded, in respect of their formulae

(but not their properties) as hydrates of the hypothetical diiodine

heptoxide, I 2 O 7. The acid commonly called 'periodic acid;

I 2 O 7. 5H 2 O, is written H 5 IO 6 (since the acid is pentabasic) and

should strictly be called hexaoxoiodic(VII) acid. It is a weak acid

and its salts are hydrolysed in solution. It can be prepared by

electrolytic oxidation of iodic(V) acid at low temperatures :

IO 3 " + 2H 2 O + OH" -» H 5 IO 6

The Aperiodic acids' and "periodates' are powerful oxidising agents

and they will oxidise manganese to manganate(VII), a reaction used

to determine small quantities of manganese in steel.

HALIDES

The rigid classification of halides into covalent and ionic can only

be an oversimplification, and the properties of the halides of a

given element can very greatly depend upon the halogen. Thus the

classification is only one of convenience.

General methods of preparation

Many salt-like halides can be prepared by the action of the hydro-

halic acid, HX, on the metal or its oxide, hydroxide or carbonate.

The halides prepared by this method are often hydrated, particu-

larly when a less electropositive metal is involved, for example zinc,

iron.