The elements of group 3 (formerly called group IIIb in North America and IIIa in Europe) include the elements scandium, yttrium, lanthanum and actinium; the lanthanoids; and the actinides. Because scandium and yttrium, despite their d electrons, behave more like lanthanoids than the metals of the transition series, they are now usually considered rare earth metals, together with lanthanum and the lanthanides. Gadolinium, which, along with its lanthanon series sibling terbium, substitutes a 4f electron with a single 5d-electron, exhibits strong ferromagnetic qualities. The atoms of the elements in group 3 have three valence electrons, although certain of the lanthanons are divalent and some, such as cerium, praseodymium and terbium, are tetravalent. Many of the actinons, exhibit variable valency.