Like barium, radium crystallizes in the body-centered cubic structure at standard temperature and pressure: the radium–radium bond distance is 514.8 picometers. Both of these values are slightly lower than those of barium, confirming periodic trends down the group 2 elements. Its color rapidly vanishes in air, yielding a black layer of radium nitride (Ra 3N 2).
Pure radium is a volatile silvery-white metal. Its physical and chemical properties most closely resemble its lighter congener barium. Radium is the heaviest known alkaline earth metal and is the only radioactive member of its group. Today, these former applications are no longer in vogue because radium's toxicity has since become known, and less dangerous isotopes are used instead in radioluminescent devices. Currently, other than its use in nuclear medicine, radium has no commercial applications formerly, it was used as a radioactive source for radioluminescent devices and also in radioactive quackery for its supposed curative powers. Radium is not necessary for living organisms, and adverse health effects are likely when it is incorporated into biochemical processes because of its radioactivity and chemical reactivity.
In nature, radium is found in uranium and (to a lesser extent) thorium ores in trace amounts as small as a seventh of a gram per ton of uraninite. Radium was isolated in its metallic state by Marie Curie and André-Louis Debierne through the electrolysis of radium chloride in 1910. They extracted the radium compound from uraninite and published the discovery at the French Academy of Sciences five days later. Radium, in the form of radium chloride, was discovered by Marie Curie and Pierre Curie in 1898. When radium decays, ionizing radiation is a product, which can excite fluorescent chemicals and cause radioluminescence. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1600 years and decays into radon gas (specifically the isotope radon-222). Pure radium is almost colorless, but it readily combines with nitrogen (rather than oxygen) on exposure to air, forming a black surface layer of radium nitride (Ra 3N 2). It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Radium is a chemical element with symbol Ra and atomic number 88.
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