Modern inorganic chemistry

230 GROUPV

elements at high temperature or in the presence of an electric dis-

charge (p. 211). It can be prepared in the laboratory by the reduction

of nitric acid and solutions of nitrates and nitrites.

The reaction between copper and nitric acid, 1 part concentrated

acid and 1 part water, gives impure nitrogen monoxide :

3Cu + 8HNO 3 -> 3Cu(NO 3 ) 2 + 4H 2 O + 2NO?

The reduction of a nitrate, for example potassium nitrate, by iron(II)

sulphate in the presence of concentrated sulphuric acid gives

reasonably pure nitrogen monoxide. The mixture is warmed and at

this temperature the nitrogen monoxide produced does not combine

with uncharged iron(II) sulphate (see below).

Industrially nitrogen monoxide is prepared by the catalytic

oxidation of ammonia as an intermediate in the manufacture of

nitric acid (p. 238). The molecule of nitrogen monoxide contains

an odd number of electrons and can be represented as

This shows the unpaired electron on the nitrogen atom ; it is in fact
"shared' over the whole molecule. Molecules such as nitrogen
monoxide which contain unpaired electrons are referred to as odd
electron molecules. The presence of the odd electron can be detected
by magnetic experiments when such substances are found to be
paramagnetic, and they are attracted into a magnetic field (see
note on p. 229). Molecules and ions containing unpaired electrons
are very weakly attracted by a magnetic field. In some cases the
total number of electrons may be even and yet the molecule may
still be paramagnetic; this is because some of the electrons are
unpaired, for example oxygen is paramagnetic. The presence of the
unpaired electron explains why, chemically, nitrogen monoxide is
more reactive than dinitrogen oxide. However, the properties of
nitrogen monoxide differ significantly from other odd electron
molecules. For example, the gaseous form is colourless although
both the liquid and solid are blue. At room temperature it shows
little tendency to dimerise, a process which would result in the
pairing of the odd electron. However, loss of this odd electron
gives the nitrosonium or nitrosyl ion, NO^. A number of salts
containing this ion are known, for example nitrosyl tetrafluoroborate,
(NO) + (BF 4 )", and nitrosyl hydrogensulphate, (NO) + (HSO 4 )".
(This last compound is formed in the lead chamber process for
sulphuric acid manufacture.)
Nitrogen oxide does show some ability to gain an electron and
when passed into a solution of sodium in liquid ammonia, the