Modern inorganic chemistry

210 GROUPV

again to give the polymerised violet allotrope. Red phosphorus may

have a structure intermediate between that of violet phosphorus

and white phosphorus, or it may be essentially similar to the violet

species.

Black phosphorus is formed when white phosphorus is heated

under very high pressure (12000 atmospheres). Black phosphorus

has a well-established corrugated sheet structure with each phos

phorus atom bonded to three neighbours. The bonding forces

between layers are weak and give rise to flaky crystals which

conduct electricity, properties similar to those ol graphite. It is

less reactive than either white or red phosphorus.

Arsenic and antimony resemble phosphorus in having several

allotropic modifications. Both have an unstable yellow allotrope.

These allotropes can be obtained by rapid condensation of the

vapours which presumably, like phosphorus vapour, contain As 4

and Sb 4 molecules respectively. No such yellow allotrope is known

for bismuth. The ordinary form of arsenic, stable at room tempera-

ture, is a grey metallic-looking brittle solid which has some power

to conduct*. Under ordinary conditions antimony and bismuth are

silvery white and reddish white metallic elements respectively.

CHEMICAL REACTIVITY

1. Reaction with air

NITROGEN

The dissociation energy of the N=N bond is very large. 946 kJ mol" \
and dissociation of nitrogen molecules into atoms is not readily
effected until very high temperatures, being only slight even at
3000 K. It is this high bond energy coupled with the absence of bond
polarity that explains the low reactivity of nitrogen, in sharp
contrast to other triple bond structures such as —C=N, —C^O,
—C^C—t. Nitrogen does, however, combine with oxygen to a
small extent when a mixture of the gases is subjected to high tempera-
ture or an electric discharge, the initial product being nitrogen

• The incorporation of minute amounts of arsenic in semi-conductors has been
  mentioned (p. 166).
  •!• Certain living systems can 'fix' atmospheric nitrogen, using a metalloenzyme
  called nitrogenase. Attempts are being made to imitate this mode of fixation by
  synthesising transition metal complexes in which molecular nitrogen, N 2 , is present
  as a ligand. The problem of easy conversion of this to (for example) NH 3 or NOJ
  remains to be solved.