Actinoids: Electronic Configuration, Oxidation States, Comparison with Lanthanoids, and Uses

The fourteen actinoids are the elements thorium (Th, $Z=90$) through lawrencium (Lr, $Z=103$), which follow actinium (Ac, $Z=89$) in the periodic table. Across this series, electrons are progressively added to the 5f subshell, giving the general outer configuration

directly analogous in form to the lanthanoids' $[\text{Xe}]4f^{1-14}5d^{0-1}6s^2$, but built on the radon core and involving the 5f (rather than 4f) subshell. As with the lanthanoids, the filling pattern is somewhat irregular across the series, since the 5f, 6d and 7s orbitals lie unusually close together in energy for the actinoids -- even closer, relatively speaking, than the corresponding 4f/5d/6s set is for the lanthanoids.

A wider range of oxidation states than the lanthanoids. This is the single most important chemical difference between the two f-block families. Because the 5f, 6d and 7s orbitals of an actinoid atom are close enough in energy that electrons from all three subshells can participate in bonding (not just the outermost $7s$, as is essentially the case for the lanthanoids' inner, poorly-bonding 4f electrons), actinoids characteristically show a much wider spread of oxidation states than the corresponding lanthanoids. While $+3$ remains a common and often the most stable state (particularly for the later, heavier actinoids, which behave more like the lanthanoids as the 5f electrons become progressively more core-like and less bonding), states of $+4$, $+5$, $+6$ and even $+7$ are also well established for the earlier actinoids: uranium, for instance, shows oxidation states from $+3$ to $+6$, with $+6$ (as in the uranyl ion, $\text{UO}_2^{2+}$) the most important industrially; neptunium and plutonium can reach $+7$ under strongly oxidizing conditions.

Comparison with lanthanoids -- summary. Both families show a steady, cumulative contraction in atomic and ionic radius across the series (an "actinoid contraction," directly analogous to lanthanoid contraction and, if anything, somewhat more pronounced, since 5f orbitals shield even more poorly than 4f orbitals). Both are electropositive metals that readily form $M^{3+}$ ions and combine with oxygen, halogens and other electronegative elements. But whereas the lanthanoids show an overwhelmingly uniform $+3$ oxidation state throughout, the actinoids show a genuinely wide and chemically significant spread of oxidation states, particularly among the earlier members (Th through Am). A further practical difference is that every actinoid is radioactive, and most of the elements beyond uranium (the so-called transuranium elements, from neptunium onward) do not occur naturally in any significant quantity; they are synthesized artificially, either by neutron capture and subsequent beta decay in a nuclear reactor, or by bombarding a heavy target nucleus with accelerated light nuclei in a particle accelerator. …