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 physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent 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 1400 deg C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point of 3300 deg C, which is the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points. 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.