Main article: Isotopes of radium Decay chain of 238U, the primordial progenitor of 226Ra Radium has a density of 5.5 g/cm 3, higher than that of barium, again confirming periodic trends the radium-barium density ratio is comparable to the radium-barium atomic mass ratio, ĭue to the two elements' similar crystal structures. Like barium and the alkali metals, 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. (p 112) This tint rapidly vanishes on exposure to air, yielding a black layer of what is probably radium nitride (Ra 3N 2). Pure radium is a volatile silvery-white metal, although its lighter congeners calcium, strontium, and barium have a slight yellow tint. 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. These applications have become obsolete owing to radium's toxicity as of 2020, less dangerous isotopes (of other elements) are instead used in radioluminescent devices. Formerly, around the 1950s, it was used as a radioactive source for radioluminescent devices and also in radioactive quackery for its supposed curative power.
As of 2014, other than its use in nuclear medicine, radium has no commercial applications. Radium is not necessary for living organisms, and its radioactivity and chemical reactivity make adverse health effects likely when it is incorporated into biochemical processes because of its chemical mimicry of calcium. In nature, radium is found in uranium ores 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 1911. 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 and Pierre Curie in 1898 from ore mined at Jáchymov. When radium decays, it emits ionizing radiation as a by-product, which can excite fluorescent chemicals and cause radioluminescence. All isotopes of radium are radioactive, the most stable isotope being radium-226 with a half-life of 1,600 years. Pure radium is silvery-white, but it readily reacts with nitrogen (rather than oxygen) upon 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 the symbol Ra and atomic number 88.
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