370 THE TRANSITION ELEMENTS
resistance to corrosion (use in chemical plant), and its retention of
these properties up to about 800 K.
The extraction of titanium is still relatively costly; first the dioxide
TiO 2 is converted to the tetrachloride TiCl 4 by heating with carbon
in a stream of chlorine; the tetrachloride is a volatile liqwd which
can be rendered pure by fractional distillation. The next stage is
costly; the reduction of the tetrachloride to the metal, with mag-
nesium, must be carried out in a molybdenum-coated iron crucible in
an atmospheric of argon at about 1100 K:
TiCl 4 -h 2Mg -» Ti 4- 2MgCl 2 : Aff = - 540 kJ moPlThe precautions stated are to avoid uptake of oxygen, nitrogen
and other impurities which render the metal brittle; the excess
magnesium and magnesium chloride can be removed by volatilisa-
tion above 1300 K.
PROPERTIESTitanium is a silver-grey metal, density 4.5 g cm~^3 , m.p. about
1950 K. When pure it is soft; presence of small amounts of impurity
make it hard and brittle, and heating with the non-metals boron,
carbon, nitrogen and oxygen gives solids which approach the
compositions TiB 2 , TiC, TiN and TiO 2 ; some of these are inter-
stitial compounds, which tend to be non-stoichiometric and harder
than the pure metal (p. 369). With hydrogen, heating gives a non-
stoichiometric hydride, which loses hydrogen at higher tempera-
tures. The metal resists attack by chlorine except at elevated tem-
peratures.
COMPOUNDS OF TITANIUM
Oxidation state + 4
In this oxidation state the titanium atom has formally lost its 3d^2
and 4s^2 electrons; as expected, therefore, it forms compounds
which do not have the characteristics of transition metal compounds,
and which indeed show strong resemblances to the corresponding
compounds of the lower elements (Si, Ge, Sn, Pb) of Group IV—the
group into which Mendeleef put titanium in his original form of
the periodic table.