When it is exposed to air, a black coating of nitride rapidly forms. It combines directly with water to form the hydroxide. It reacts with acids to form the commercially important chloride and bromide. Mixed with a phosphor such as zinc sulfide, radium compounds are used in luminous paints. Radium is a rare metal.
Radium is present in all uranium minerals and is widely distributed in small amounts.
It is recovered as the bromide by an involved chemical process. The small amount of the element present in any ore and the difficulty of extraction make it expensive. Radium was discovered in 1898 by Pierre and Marie Curie in pitchblende given them by Austria after the uranium salts had been removed for use in glass manufacture. They had earlier found polonium in a similar sample. The Columbia Encyclopedia, Sixth Edition.