least milligram quantities, thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter. Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points.

The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually thorium dioxide ; even the purest thorium specimens usually contain about a tenth of a per cent of the dioxide. Experimental measurements