Tantalum, a rare metallic element which was discovered in 1802, in some rare minerals known as tantalite and yttrotantalite. The metal itself has been obtained as a black powder of specific gravity 10:8, which burns if heated in air or in chlorine gas. It is unacted on by acids, with the sole exception of hydrofluoric acid. It forms a number of oxides, one of which - the pentoxide Ta2O5 - acts the part of an acid oxide, uniting with bases to form salts known as tantalates. It forms also a nitride TaN, a metallic-like compound. The atomic weight of tantalum is still uncertain, but it appears to lie between 182 and 183.