In general, the third
ionisation enthalpies are quite high Also the high values for third
ionisation enthalpies of copper, nickel and zinc indicate why it is
difficult to obtain oxidation state greater than two for these elements Although ionisation enthalpies give some guidance concerning the
relative stabilities of oxidation states, this problem is very complex and
not amenable to ready generalisation One of the notable features of a transition elements is the great variety
of oxidation states these may show in their compounds