A lanthanide is any member of a group (called the lanthanum series) of rare, metallic elements with atomic numbers from 57 (lanthanum) to 71 (lutetium) inclusive, in which the 4f orbitals are being filled. The lanthanides are: lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Because their electronic structures are very similar, differing only in inner orbitals, the properties of the lanthanides are all very similar. The lanthanides are all reactive metals resembling scandium, forming trivalent salts and ligand complexes. Cerium, praseodymium, and terbium also form tetravalent compounds, and europium, ytterbium, and samarium form divalent ones. All form divalent ionic hydrides and sulfides.
There is a decrease (the lanthanide contraction) in ionic radii through the series, so that the third row transition element following the lanthanides have ionic radii almost identical to those of their analogues in the second row, and hence have similar properties.
The lanthanides are separated by chromatography and ion-exchange resins. They are used in alloys, including misch metal; and their compounds (mixed or separately) are used as abrasives, for making glasses and ceramics, as "getters," as catalysts in the petroleum industry, and to make phosphors, lasers, and microwave devices.
Except for promethium, the lanthanides are not uncommon – the normal source is monazite which contains 90% La, Ce, Pr, Nd in its lanthanide contents.
See also rare earth elements.