A Short Discussion on the Chemistry of Actinides

The lanthanides have, as known, very similar chemical properties across the series. Writing them in a single separate line in the periodic chart intends to convey this information to the reader. 

In Fig. 1, we have plotted the oxidation numbers of the actinides and of the lanthanides. We see that for the lanthanides the valence 3 is the most stable valence throughout the series. There are exceptions Ce displays for instance tetravalency in many compounds Eu and Yb display divalency. These exceptions are understood e.g., Eu and Yb are at the half-filling and at the filling of the 4f shell, which are stable electronic configurations. There is a tendency for both to share just the two outer 5 s electrons in bonding, displaying therefore, divalency, and preserve these stable configurations. 

On the contrary, there is a spread of oxidation numbers for the light actinides (at least up to Cm), which, for Pu and Np, range from 3 to 7 After Cm, however, the trivalent oxidation state is always met, and this second half of the actinide series approaches more the behaviour of the lanthanides. 

the implications about the chemical behaviour derived from a Periodic Chart in which the actinides are all placed in one line may be somewhat misleading. Rather, it appears that we should distinguish two parts in the series one up to Cm ( light actinides ), another one from Cm on ( heavy actinides ). 

In Fig. 1, the valences of the transition 3d-, 4d-, 5d-series are also plotted. As for what regards the spread of valences, an interesting observation is that, at least for the hght actinides (if not for the whole series), there is more similarity between the actinides md the d-transition elements than the actinides and lanthanides. 

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