The lanthanide elements are the fifteen elements ranging from lanthanum to lutetium. They are also known as the rare earths, although the term 'earth' should really be applied only to the metals' oxides, and none of the elements are as rare as they were once thought to be. Scandium and yttrium are sometimes included in this series, and lanthanum is sometimes excluded. The lanthanides were so-named because they possess similar properties to lanthanum, and would be fitting members of lanthanum's group in terms of typical valency, but they are designated a separate division on the grounds that their valencies are even more variable than those of the d-block transition metals - their electrons fill a different shell.
The lanthanides are soft, silvery-white metals, the hardness of which increases left-to-right across the series. They show variable valency, although they all form trivalent compounds.
All of the metals are reactive. They tarnish on exposure to air, burn readily in oxygen or air, and react, when heated, with hydrogen, the halogens, and most of the other non-metals. They are attacked by water and by dilute acids. Europium is the most reactive member of the series.
Misch metal, an alloy of iron and lanthanides is pyrophoric: it sparks when scratched, and so is of use in cigarette lighter flints.
Many of the lanthanides absorb neutrons and are used in control rods in nuclear reactors, where nuclear chain reactions are perpetuated by neutrons.
Compared with most metals, none of the lanthanides are particularly good conductors of electricity.
