Why do heavy lanthanides resist further oxidation in air?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Heavy lanthanides resist further oxidation in air because they form a protective, coherent C-type oxide ( $R_{2} O_{3}$ ) coating on their surface when exposed to oxygen. This layer acts as a barrier that prevents further oxidation, similar to the behavior of aluminum.1

In contrast, light lanthanides form a different, hexagonal A-type oxide structure that reacts with water vapor in the air to produce an oxyhydroxide. This reaction causes the surface coating to spall (flake off), which exposes fresh metal and allows oxidation to continue. Because of this protective C-type layer, heavy lanthanides—along with scandium and yttrium—can remain in open air for years without significant oxidation.1