The Rare Earths "Rare earths" is a highly inaccurate name. The word "rare" comes from the fact that the lanthanides were originally discovered in rare minerals, but the elements themselves are not especially scarce; some are quite common. "Earth" is an old chemical term meaning oxide, or a compound with oxygen; the rare earths, which are actually metals, were first seen in this form. The chemical properties of the rare earths, based on their matching valence electrons, are almost identical. How do you tell them apart? They have slightly different atomic weights, of course, but more importantly, some of them have other interesting properties that come from the differences in their 4f electrons. For example, gadolinium (Gd) is the only rare earth that's ferromagnetic--that is, it sticks to magnets, the way iron does. Lanthanum is the only superconductor among them; at very low temperatures, it loses all resistance to the flow of electricity.

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