half of the series frequently exhibit higher oxidation states. For example, the maximum oxidation state increases from + in Th to + , + and + respectively in Pa, U and Np but decreases in succeeding elements (Table . ). The actinoids resemble the lanthanoids in having more compounds in + state than in the + state.
However, + and + ions tend to hydrolyse. Because the distribution of oxidation states among the actinoids is so uneven and so different for the former and later elements, it is unsatisfactory to review their chemistry in terms of oxidation states. . .
Electronic Configurations . . Ionic Sizes . .
Oxidation States The actinoid metals are all silvery in appearance but display a variety of structures. The structural variability is obtained due to irregularities in metallic radii which are far greater than in lanthanoids. . .
General Characteristics and Comparison with Lanthanoids Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Table . : Oxidation States of Actinium and Actinoids The actinoids are highly reactive metals, especially when finely divided. The action of boiling water on them, for example, gives a mixture of oxide and hydride and combination with most non metals takes place at moderate temperatures. Hydrochloric acid attacks all metals but most are slightly affected by nitric acid owing to the formation of protective oxide layers; alkalies have no action.
The magnetic properties of the actinoids are more complex than those of the lanthanoids. Although the variation in the magnetic susceptibility of the actinoids with the number of unpaired f electrons is roughly parallel to the corresponding results for the lanthanoids, the latter have higher values. It is evident from the behaviour of the actinoids that the ionisation enthalpies of the early actinoids, though not accurately known, but are lower than for the early lanthanoids. This is quite reasonable since it is to be expected that when f orbitals are beginning to be occupied, they will penetrate less into the inner core of electrons.
The f electrons, will therefore, be more effectively shielded from the nuclear charge