The Lanthanoids

The $f$-block of the periodic table is made up of two series that sit below the main body of the table: the lanthanoids, the fourteen elements that come right after lanthanum, and the actinoids, the fourteen elements that come right after actinium.

Because lanthanum behaves almost identically to the elements that follow it, chemists conventionally fold it into the lanthanoid discussion as well, and the whole set — La plus the fourteen elements after it — is given the collective symbol $\text{Ln}$. The same convention applies on the actinoid side, where actinium is usually discussed together with the fourteen elements that follow it.

Why the Lanthanoids Are Treated as a Tight Family

The lanthanoids resemble each other far more closely than the members of an ordinary $d$-block transition series resemble one another. A key reason is that, across the whole series, there is essentially only one stable oxidation state in solution and in most compounds. Because the atoms are all so alike in behaviour, the lanthanoid series becomes an unusually clean case study: it lets you see, in isolation, what a small, steady change in atomic size and nuclear charge does to chemical behaviour along a series of otherwise very similar elements — without the complications that come from a wide swing in oxidation state.

The actinoids, by contrast, are chemically far more complicated. Two things make them harder to study: they show a much wider spread of oxidation states than the lanthanoids, and their radioactivity introduces extra practical difficulties in handling and investigating them. For this reason, the two $f$-block series are best discussed as two separate stories rather than as one.

What This Section Covers …