radium (Ra), radioactive chemical element, heaviest of the alkaline-earth metals of main Group IIa of the periodic table. Radium is a silvery white metal that does not occur free in nature.

Radium was discovered (1898) by Pierre Curie, Marie Curie, and an assistant, G. Bémont, after Mme Curie had observed that the radioactivity of pitchblende was four or five times greater than that of the uranium it contained and not fully explained on the basis of radioactive polonium, which she had just discovered in pitchblende residues. The new, powerfully radioactive substance followed the behaviour of barium, but because its chloride was slightly more insoluble it could be concentrated by fractional crystallization. By 1902, one-tenth gram of pure radium chloride was prepared by refining several tons of pitchblende residues, and by 1910 Mme Curie and André-Louis Debierne had isolated the metal itself.



When concentrated, radium glows in the dark. Because of this property, it was once mixed with a paste of zinc sulfide to make a self-luminescent paint for watch, clock, and instrument dials. During the 1930s it was found, however, that exposure to radium posed a serious hazard to health: a number of the workers who routinely used the radium-containing luminescent paint developed anemia and, in some cases, bone cancer. The practice of employing radium in luminescent coatings was halted after the high toxicity of the material was recognized.



atomic number88stablest isotope226melting pointabout 700° Cboiling pointabout 1,737° Cspecific gravityabout 5valence2electronic config.2-8-18-32-18-8-2 or (Rn)5oxidation state+2electronic config.[Rn]7s2