Modern inorganic chemistry

THE TRANSITION ELEMENTS 369

Because the metal structure is 'locked' by these atoms, the result-

ing compound is often much harder than the original metal, and

some of the compounds are therefore of industrial importance (see

under iron). Since there is a definite ratio of holes to atoms, filling

of all the holes yields compounds with definite small atom-metal

atom ratios; in practice, all the holes are not always filled, and com-

pounds of less definite composition (non-stoichiometric compounds)

are formed.

The metals: other properties

Adsorption of gases on to transition metal surfaces is important,

and transition metals or alloys are often used as heterogeneous

catalysts.

The reactivity of the transition metals towards other elements

varies widely. In theory, the tendency to form other compounds

both in the solid state (for example reactions to form cations) should

diminish along the series; in practice, resistance to reaction with

oxygen (due to formation of a surface layer of oxide) causes chromium

(for example) to behave abnormally; hence regularities in reactivity

are not easily observed. It is now appropriate to consider the indi-

vidual transition metals.

SCANDIUM

Scandium is not an uncommon element, but is difficult to extract.

The only oxidation state in its compounds is + 3, where it has

formally lost the 3d^1 4s^2 electrons, and it shows virtually no transition

characteristics. In fact, its chemistry is very similar to that of alu-

minium (for example hydrous oxide Sc 2 O 3 , amphoteric; forms a

complex [ScF 6 ]^3 ~ ; chloride ScCl 3 hydrolysed by water).

TITANIUM

THE ELEMENT

Titanium is not a rare element; it is the most abundant transition
naetal after iron, and is widely distributed in the earth's surface,
mainly as the dioxide TiO 2 and ilmenite FeTiO 3. It has become of
commercial importance since World War II mainly because of its
high strength-weight ratio (use in aircraft, especially supersonic), its