Whose work first isolated the rare earth elements and helped define them as a chemical family?

Carl Gustaf Mosander made major contributions to the chemistry of the rare earth elements. By careful chemical separation of minerals, he identified lanthanum in 1839 and later helped separate erbium and terbium from complex mixtures. The rare earths were notoriously difficult to distinguish because they have very similar chemical properties. Mosander’s work helped reveal that these elements formed a family with subtle differences, later crucial to modern materials science. The rare earths eventually became essential in magnets, electronics, and lasers.