122 GROUPS I AND II
charge, has also some peculiar properties in solution, which are
examined later (p. 134).OCCURRENCE AND EXTRACTION
The alkali metals of Group I are found chiefly as the chlorides (in
the earth's crust and in sea water), and also as sulphates and
carbonates. Lithium occurs as the aluminatesilicate minerals,
spodumene and lepidolite. Of the Group II metals (beryllium to
barium) beryllium, the rarest, occurs as the aluminatesilicate, beryl \
magnesium is found as the carbonate and (with calcium) as the
double carbonate dolomite', calcium, strontium and barium all
occur as carbonates, calcium carbonate being very plentiful as
limestone.
The general characteristics of all these elements generally preclude
their extraction by any method involving aqueous solution. For the
lighter, less volatile metals (Li, Na, Be, Mg, Ca) electrolysis of a
fused salt (usually the chloride), or of a mixture of salts, is used.
The heavier, more volatile metals in each group can all be similarly
obtained by electrolysis, but it is usually more convenient to take
advantage of their volatility and obtain them from their oxides or
chlorides by displacement, i.e. by general reactions such as3M 2 O + 2Mm -* M 2 mO 3 4- 6M|MCI + M^1 ~» M!C1 + M|Thus potassium is obtained by heating potassium chloride with
sodium, and barium by reduction of barium oxide with aluminium.
Sodium is important in many technical processes and is therefore
prepared in considerable quantity. Almost all of it is now made by
electrolysis of the fused sodium chloride, using the Downs cell (see
Figure 6.1). The graphite anode is cylindrical and is surrounded by
the steel gauze diaphragm and the concentric cylindrical cathode
(also of steel). The electrolyte is usually a mixture of sodium chloride
and calcium chloride; the latter is added to reduce the m.p. of the
sodium chloride to approximately 800 K. (Some calcium is therefore
liberated with the sodium.) The gap between anode and cathode is
kept as small as possible to reduce resistance: the heat developed
by the current maintains the temperature of the cell. Chlorine is set
free at the anode surface, rises into the nickel cone and can be
collected. Sodium, liberated at the cathode, is prevented by the
diaphragm from passing into the anode region; the molten sodium
collects under the circular hood and rises up the pipe, being assisted