Lanthanons (also called lanthanoids or lanthanides) follow lanthanum in group 3; lanthanum is usually considered a member of this series. Excluding lanthanum, the 14-member series, known collectively as the rare earth metals, consists of cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. In the elements of the lanthanon series the inner f orbital, the 4f subshell, is filled as atomic number increases. The lanthanons, excepting promethium, which is radioactive, are not particularly rare, occurring in many minerals but chiefly in monazite sands. Pure lanthanons are silvery metals with high melting points. All but samarium, europium and ytterbium tarnish slowly in air, these exceptions being much more reactive toward oxygen or moisture.