Lanthanide contraction in the context of "Scandium group"

A lanthanide (/ˈlænθənaɪd/) is any of the 14 metallic chemical elements with atomic numbers 57–70, from lanthanum through ytterbium. The IUPAC recommended standard name of this series is lanthanoid (/ˈlænθənɔɪd/).In the periodic table, they fill the 4f orbitals. Lutetium (element 71) is also sometimes considered a lanthanide, despite being a d-block element and a transition metal.

The informal chemical symbol Ln is used in general discussions of lanthanide chemistry to refer to any lanthanide. All but one of the lanthanides are f-block elements, corresponding to the filling of the 4f electron shell. Lutetium is a d-block element (thus also a transition metal), and on this basis its inclusion has been questioned; however, like its congeners scandium and yttrium in group 3, it behaves similarly to the other 14. The term rare-earth element or rare-earth metal is often used to include the stable group 3 elements Sc, Y, and Lu in addition to the 4f elements. All lanthanide elements form trivalent cations, Ln, whose chemistry is largely determined by the ionic radius, which decreases steadily from lanthanum (La) to lutetium (Lu).