Nitrogen is a colourless, odourless, tasteless, non-toxic gas. It can be
condensed into a colourless liquid, which can in turn be compressed into a
colourless, crystalline solid. Nitrogen exists in two natural isotopic forms,
and four radioactive isotopes have been artificially prepared. Nitrogen melts
at -210.01?C (-346.02?F), boils at -195.79?C (-320.42?F), and has a density of
1.251 g/litre at 0?C (32?F) and 1 atmosphere pressure. The atomic weight of
nitrogen is 14.007.
Nitrogen is obtained from the atmosphere by passing air over heated
copper or iron. The oxygen is removed from the air, leaving nitrogen mixed with
inert gases. Pure nitrogen is obtained by fractional distillation of liquid
air; because liquid nitrogen has a lower boiling point than liquid oxygen, the
nitrogen distills off first and can be collected.
Nitrogen combines with other elements only at very high temperatures or
pressures. It is converted to an active form by passing through an electric
discharge at low pressure. The nitrogen so produced is very active, combining
with alkali metals to form azides; with the
vapour of zinc, mercury, cadmium, and arsenic to form nitrides; and with many hydrocarbons to form hydrocyanic acid and
cyanides, also known as nitriles. Activated nitrogen returns to ordinary
nitrogen in about one minute.
In the combined state nitrogen takes part in many reactions; it forms so
many compounds that the American chemist Edward Franklin created a systematic
scheme of compounds containing nitrogen in place of oxygen. In compounds
nitrogen exists in all the valence states between -3 and +5. Ammonia,
hydrazine, and hydroxylamine represent compounds in which the valence of
nitrogen is -3, -2, and -1, respectively. Oxides of nitrogen represent nitrogen
in all the positive valence states.