General Characteristics and Comparison with Lanthanoids

Appearance and structure

The actinoid metals are all silvery in appearance, but unlike the lanthanoids, they display considerable variety in their crystal structures. This structural variability traces back to irregularities in the metallic radii across the series, which are far greater in the actinoids than in the lanthanoids.

Reactivity

The actinoids are highly reactive metals, and this reactivity is especially pronounced when the metal is finely divided. Boiling water reacts with them to give a mixture of the oxide and the hydride, and they combine with most non-metals even at moderate temperatures. Hydrochloric acid attacks every metal in the series, but most actinoids are only slightly affected by nitric acid, because a protective layer of oxide forms on the surface and shields the metal beneath it. Alkalies, by contrast, have essentially no action on them.

Magnetic properties

The magnetic behaviour of the actinoids is more complicated than that of the lanthanoids. The way magnetic susceptibility varies with the number of unpaired $5f$ electrons follows a pattern roughly parallel to the corresponding trend in the lanthanoids' $4f$ electrons — but the lanthanoids reach distinctly higher magnetic-moment values than the actinoids do.

Ionisation enthalpies

The ionisation enthalpies of the early actinoids are not known with great accuracy, but the evidence available shows they run lower than those of the corresponding early lanthanoids. This is reasonable on physical grounds: as the $5f$ orbitals begin to be occupied, they penetrate less deeply into the atom's inner electron core than the $4f$ orbitals do. Consequently, the $5f$ electrons are shielded from the nuclear charge more effectively than the $4f$ electrons of the analogous lanthanoids, so the outer electrons are held less firmly — which is exactly why they are more available for bonding in the actinoids.

Comparison with the lanthanoids

Setting the actinoids and lanthanoids side by side across all these characteristics, a clear pattern emerges: lanthanoid-like behaviour does not really appear in the actinoids until the second half of the series. Even so, the earlier actinoids already resemble the lanthanoids in two more general ways — they closely resemble one another, and their properties vary gradually across the series without any accompanying change in oxidation state. …