Lanthanoids and actinoids

The f-block has two series. The lanthanoids (cerium to lutetium) fill the 4f subshell; the actinoids (thorium to lawrencium) fill the 5f.

Lanthanoids mostly show the +3 oxidation state. A few show others when that gives a stable f⁰, f⁷ or f¹⁴ arrangement: Ce⁴⁺ (f⁰) and Tb⁴⁺; and Eu²⁺ and Yb²⁺.

Actinoids show a much wider range, from +3 up to +7, because the 5f, 6d and 7s levels are close in energy. Their chemistry is less regular. All of them are radioactive, and many are made only in nuclear reactors.

The most important idea is the lanthanoid contraction. Going from lanthanum to lutetium, the atoms and ions get steadily smaller. Each step adds a proton to the nucleus and an electron to the 4f subshell, but 4f electrons shield very poorly: they are a weak guard. So the nucleus pulls the outer electrons in more and more tightly.

Its effects:

• The 4d and 5d elements of a group end up almost the same size. Zirconium and hafnium are so alike in size and chemistry that they are hard to separate.
• The lanthanoid hydroxides become less basic along the series.

The actinoids show the same effect, an actinoid contraction, and it is greater from element to element, because 5f electrons shield even more poorly.

A use: mischmetall is an alloy of about 95% lanthanoid metals with iron and traces of other elements. It is used with magnesium to make bullets, shells and lighter flints.