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The Collaborative International Dictionary of English v.0.48 (gcide)

Radium \Ra`di*um\ (r[=a]"d[i^]*[u^]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}.

Note: 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]