Thorium was originally assigned a position in Group IV of the periodic table. Because of its atomic weight, valence, etc., it is now considered to be the second member of the actinide series of elements. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The purest specimens often contain several tenths of a per cent of the oxide. High-purity thorium has been made.
Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn.
Thorium is dimorphic, changing at 14000C from a cubic to a body- centered cubic structure. Thorium oxide has a melting point of 33000C and is the highest of all oxides. Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be carefully handled. The principal use of thorium has been in the preparation of the Welsbach mantle, used for portable gas lights.