GROUP VI 265
SULPHURThe structures of sulphur in solid, liquid and gaseous phases are
complicated. Rhombic sulphur is the solid allotrope stable at room
temperature. It is yellow, readily soluble in carbon disulphide, from
which it can be crystallised, and has a density of 2.06 g cm"^3. Above
369 K, the transition temperature, rhombic sulphur is no longer
stable, slowly changing to monoelinic sulphur, and if rhombic
sulphur is melted, allowed to partly solidify, and the remaining
molten sulphur is poured off, there remain long needle-like crystals
(almost colourless) of monoelinic sulphur, density 1.96 g cm~^3. A
good specimen of monoelinic sulphur can be prepared by crystal-
lising a concentrated solution of sulphur in xylene, taking care to
keep the temperature above 368 K. On standing at room tempera-
ture, monoelinic sulphur slowly changes to the rhombic form. Both
these allotropes contain S 8 molecules with rings of eight sulphur
atoms.,-.
When sulphur is melted viscosity changes occur as the temperature
is raised. These changes are due to the formation of long-chain
polymers (in very pure sulphur, chains containing about 100 000
atoms may be formed). The polymeric nature of molten sulphur
can be recognised if molten sulphur is poured in a thin stream into
cold water, when a plastic rubbery mass known as plastic sulphur
is obtained. This is only slightly soluble in carbon disulphide, but
on standing it loses its plasticity and reverts to the soluble rhombic
form. If certain substances, for example iodine or oxides of arsenic,
are incorporated into the plastic sulphur, the rubbery character can
be preserved.
Colloidal sulphur is produced by careful addition of acid to sodium
thiosulphate solution.
SELENIUM
Like sulphur, selenium exists in a number of allotropic forms. These
include both crystalline, rhombic and monoelinic modifications