radium (Ra) /rā″dē·əm/ [L, radius, ray] , a radioactive metallic element of the alkaline earth group. Its atomic number is 88. Four radium isotopes occur naturally and have different atomic masses: 223, 224, 226, and 228. The isotope with atomic mass 226 is the most abundant. It is formed by the disintegration of uranium 238, has a half-life of 1620 years, and decays by alpha emission to form radon 222. Radium occurs in the uranium minerals carnotite and pitchblende, which contain about 3 × 10−7 g of radium per g of uranium. Radium has been used extensively as a radiation source in the treatment of cancer but is gradually being replaced in such therapy by cobalt and cesium.
