The lanthanide or lanthanoid (IUPAC nomenclature)[1] series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium.[2][3][4] These fifteen elements, along with chemically similar scandium and yttrium, are often collectively known as the rare earth elements. The informal chemical symbol Ln is used in general discussions of lanthanide chemistry. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell; lutetium, a d-block element, is also generally considered to be a lanthanide due to its chemical similarities with the other fourteen. All of the lanthanide elements form trivalent cations, Ln3+, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum to lutetium. source: http://en.wikipedia.org/wiki/Lanthanoid