Modern inorganic chemistry

GROUP VI 305

selenic(IV) (selenious) acid H 2 SeO 3 , a dibasic acid forming two

series of salts. Both the acid and its salts are fairly good oxidising

agents, oxidising (for example) sulphur dioxide and hydrogen iodide.

Selenium trioxide, SeO 3 , is a white deliquescent solid which has

never been obtained completely pure. When selenic acid(Vl),

H 2 SeO 4 , is dehydrated a mixture of selenium dioxide and trioxide is

obtained and oxygen is evolved. Selenic(VI) acid H 2 SeO 4 is formed

when selenium trioxide is dissolved in water and is a strong dibasic

acid. It is a more powerful oxidising agent than sulphuric acid and

will, for example, oxidise hydrochloric acid evolving chlorine.

Oxides and oxo-acids of tellurium

Tellurium dioxide, TeO 2 , is a white non-volatile solid obtained

when tellurium is burnt in air. It is only slightly soluble in water but

dissolves in alkalis to form salts.

Tellurium trioxide, TeO 3 , is an orange yellow powder made by

thermal decomposition of telluric(VI) acid Te(OH) 6. It is a strong

oxidising agent which will, like H 2 SeO 4 , oxidise hydrogen chloride

to chlorine. It dissolves in hot water to give telluric(VI) acid. This is

a weak acid and quite different from sulphuric and selenic acids.

Two series of salts are known.

HALIDES

Oxygen halides are dealt with in Chapter 11, p. 334. Sulphur,

selenium and tellurium form many halides, and only a brief intro-

duction to the subject is given here.

Fluorides

All three elements form gaseous hexafluorides by the direct combina-

tion of the elements. They all have octahedral structures

X = S, Se or Te.