Ra`di*um(r"d*m), n. [NL., fr. L. radius ray.] (Chem.) An intensely radioactive metallic element found (combined) in minute quantities in pitchblende, and various other uranium minerals. Symbol, Ra; atomic weight, 226.4. Radium was discovered by M. and Mme. Curie, of Paris, who in 1902 separated compounds of it by a tedious process from pitchblende. Its compounds color flames carmine and give a characteristic spectrum. It is divalent, resembling barium chemically. The main isotope of radium found in pitchblende, radium-226, has a half-life of 1620 years, decaying first by alpha emission to radon. Radium preparations are remarkable for maintaining themselves at a higher temperature than their surroundings, and for their radiations, which are of three kinds: alpha rays, beta rays, and gamma rays (see these terms). The beta and gamma rays seen in radium preparations are in fact due to disintegration of decay products of radium rather than the radium itself. By reason of these rays they ionize gases, affect photographic plates, cause sores on the skin, and produce many other striking effects. Their degree of activity depends on the proportion of radium present, but not on its state of chemical combination or on external conditions. The radioactivity of radium is therefore an atomic property, and is due to an inherent instability of the atomic nucleus which causes its decay in a process whose rate is first order. The disintegration of the radium nucleus is only the first in a series of nuclear disintegrations leading to production of a series of elements and isotopes. The chain has at least seven stages; the successive main products have been studied and are radon, a gaseous radioactive element belonging chemically to the inert noble gas series (originally called radium emanation or exradio, radium A, radium B, radium C, etc. The successive products are unstable isotopes of several different elements, each with an atomic weight a little lower than its predecessor. Lead is the stable end product. At the same time, the light gas helium is formed, being generated when the expelled alpha particles (positively charged helium nuclei) acquire electrons. Radium, in turn, is formed in the pitchblende ore by a slow disintegration of uranium. Natural radium and also an isotope (radium-228, also called mesothorium I) formed by the decay of thorium, were at one time used to make a luminous paint for watch dials, until the danger of the radioactivity became fully appreciated, and use of such material in watches was discontinued. See also mesothorium.
[Webster 1913 Suppl. +PJC]
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