Modern inorganic chemistry

GROUPV 209

These gases leave the furnace at about 600 K. pass through electro-
static precipitators to remove dust, and the phosphorus is then
condensed out.

ARSENIC, ANTIMONY AND BISMUTH

Each of these elements occurs naturally as a sulphide ore: arsenic

as realgar As 4 S 4 , orpiment As 4 S 6 and arsenical pyrites with approxi-

mate formula FeAsS; antimony as stibnite Sb 2 S 3 ; and bismuth as

Bi 2 S 3.

The method of extraction is similar for each element involving

first the roasting of the sulphide ore when the oxide is produced, for

example

Sb 2 S 3 + 5O 2 -> Sb 2 O 4 + 3SO 2

followed by reduction of the oxide with carbon, for example

As 4 O 6 + 6C -> As 4 + 6COt

PROPERTIES OF THE ELEMENTS

The main physical properties of these elements have been given in

Table 9.1.

ALLOTROPES

Solid phosphorus, arsenic and antimony exist in well known allo-
tropic modifications. Phosphorus has three main allotropic forms,
white, red and black. White phosphorus is a wax-like solid made
up of tetrahedral P 4 molecules with a strained P—P—P angle of
60°; these also occur in liquid phosphorus. The reactivity of white
phosphorus is attributed largely to this strained structure. The
rather less reactive red allotrope can be made by heating white
phosphorus at 670K for several hours; at slightly higher tempera-
tures, - 690 K, red phosphorus sublimes, the vapour condensing to
reform white phosphorus. If, however, red phosphorus is heated in
a vacuum and the vapour rapidly condensed, apparently another
modification, violet phosphorus, is obtained. It is probable that violet
phosphorus is a polymer of high molecular weight which on heating
breaks down into P 2 molecules. These on cooling normally dimerise
to form P 4 molecules, i.e. white phosphorus, but in vacua link up