262 GROUP V!SELENIUM AND TELLURIUMSelenium and tellurium occur naturally in sulphide ores, usually as
an impurity in the sulphide of a heavy metal. They are recovered
from the flue dust produced when the heavy metal sulphide is roasted.POLONIUM
This is a radioactive element. It occurs in minute traces in barium
and thorium minerals, but it can be produced by irradiation of bis-
muth in a nuclear reactor. (The study of its chemistry presents great
difficulty because of its intense a radiation).ALLOTROPES
Oxygen, sulphur and selenium are known to exist in more than one
allotropic form.OXYGEN
This exists in two allotropic forms, oxygen, O 2 and ozone, O 3.
Oxygen is a colourless gas which condenses to a pale blue liquid,
b.p. 90 K, which is markedly paramagnetic indicating the presence of
unpaired electrons (p. 229). Simple valence bond theory (as used in
this book) would indicate the structure
:'p: q: i.e. 0=0which accounts for the high oxygen-oxygen bond strength (bond
dissociation energy, 49 kJ mol"^1 ). but does not explain the para-
magnetism. The molecular orbital theory of bonding, however,
suggests not only a doubly bonded structure but also two molecular
orbitals (i.e. orbitals of the complete O 2 molecule) of equal energy
each containing one electron, and this satisfactorily explains both the
high bond strength and paramagnetism.
Oxygen, like nitrogen oxide, NO, shows little tendency to dimerise
although the presence of the unstable, weakly bonded species,
tetratomic oxygen O 4 , has been reported as a constituent of liquid
oxygen.
Ozone, O 3 , is found in trace quantities in the upper atmosphere
where it is believed to be formed by the photochemical dissociation
of oxygen molecules by the intense ultra-violet light from the sun;