Radium
Radium is a chemical element; it has symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen upon exposure to air, forming a black surface layer of radium nitride. All isotopes of radium are radioactive, the most stable isotope being radium-226 with a half-life of 1,600 years. When radium decays, it emits ionizing radiation as a by-product, which can excite fluorescent chemicals and cause radioluminescence. For this property, it was widely used in self-luminous paints following its discovery. Of the radioactive elements that occur in quantity, radium is considered particularly toxic, and it is carcinogenic due to the radioactivity of both it and its immediate decay product radon as well as its tendency to accumulate in the bones.
Radium, in the form of radium chloride, was discovered by Marie and Pierre Curie in 1898 from ore mined at Jáchymov. They extracted the radium compound from uraninite and published the discovery at the French Academy of Sciences five days later. Radium was isolated in its metallic state by Marie Curie and André-Louis Debierne through the electrolysis of radium chloride in 1910, and soon afterwards the metal started being produced on larger scales in Austria, the United States, and Belgium. However, the amount of radium produced globally has always been small in comparison to other elements, and by the 2010s, annual production of radium, mainly via extraction from spent nuclear fuel, was less than 100 grams.
In nature, radium is found in uranium ores in quantities as small as a seventh of a gram per ton of uraninite, and in thorium ores in trace amounts. 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, due to them both being group 2 elements. As of 2018, other than in nuclear medicine, radium has no commercial applications. Formerly, from the 1910s to the 1970s, it was used as a radioactive source for radioluminescent devices and also in radioactive quackery for its supposed curative power. In nearly all of its applications, radium has been replaced with less dangerous radioisotopes, with one of its few remaining non-medical uses being the production of actinium in nuclear reactors.
Bulk properties
Radium is the heaviest known alkaline earth metal and is the only radioactive member of its group. Its physical and chemical properties most closely resemble its lighter congener, barium.Pure radium is a volatile, lustrous silvery-white metal, even though its lighter congeners calcium, strontium, and barium have a slight yellow tint. Radium's lustrous surface rapidly becomes black upon exposure to air, likely due to the formation of radium nitride. Its melting point is either or and its boiling point is ; however, this is not well established. Both of these values are slightly lower than those of barium, confirming periodic trends down the group 2 elements.
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.
Radium has a density of 5.5 g/cm, higher than that of barium, and the two elements have similar crystal structures.
Isotopes
Radium has 33 known isotopes with mass numbers from 202 to 234, all of which are radioactive. Four of these – Ra, Ra, Ra, and Ra – occur naturally in the decay chains of primordial thorium-232, uranium-235, and uranium-238. These isotopes nevertheless still have half-lives too short to be primordial radionuclides, and only exist in nature from these decay chains.Together with the mostly artificial Ra, which occurs in nature only as a decay product of minute traces of neptunium-237,
these are the five most stable isotopes of radium. All other 27 known radium isotopes have half-lives under two hours, and the majority have half-lives under a minute. Of these, Ra also occurs as a Np daughter, and Ra and Ra would be produced by the still-unobserved double beta decay of natural radon isotopes. At least 12 nuclear isomers have been reported, the most stable of which is radium-205m with a half-life between 130~230 milliseconds; this is still shorter than twenty-four ground-state radium isotopes.
Ra is the most stable isotope of radium and is the last isotope in the decay chain of uranium-238 with a half-life of over a millennium; it makes up almost all of natural radium. Its immediate decay product is the dense radioactive noble gas radon, which is responsible for much of the danger of environmental radium. It is 2.7 million times more radioactive than the same molar amount of natural uranium, due to its proportionally shorter half-life.
A sample of radium metal maintains itself at a higher temperature than its surroundings because of the radiation it emits. Natural radium emits mostly alpha particles, but other steps in its decay chain emit alpha or beta particles, and almost all particle emissions are accompanied by gamma rays.
Experimental nuclear physics studies have shown that nuclei of several radium isotopes, such as Ra, Ra and Ra, have reflection-asymmetric shapes. In particular, this experimental information on radium-224
has been obtained at ISOLDE using a technique called Coulomb excitation.
Chemistry
Radium only exhibits the oxidation state of +2 in solution. It forms the colorless Ra cation in aqueous solution, which is highly basic and does not form complexes readily. Most radium compounds are therefore simple ionic compounds, though participation from the 6s and 6p electrons is expected due to relativistic effects and would enhance the covalent character of radium compounds such as RaF and RaAt. For this reason, the standard electrode potential for the half-reaction Ra + 2e → Ra is −2.916 V, even slightly lower than the value −2.92 V for barium, whereas the values had previously smoothly increased down the group. The values for barium and radium are almost exactly the same as those of the heavier alkali metals potassium, rubidium, and caesium.Compounds
Solid radium compounds are white as radium ions provide no specific coloring, but they gradually turn yellow and then dark over time due to self-radiolysis from radium's alpha decay. Insoluble radium compounds coprecipitate with all barium, most strontium, and most lead compounds.Radium oxide is poorly characterized, as the reaction of radium with air results in the formation of radium nitride. Radium hydroxide is formed via the reaction of radium metal with water, and is the most readily soluble among the alkaline earth hydroxides and a stronger base than its barium congener, barium hydroxide. It is also more soluble than actinium hydroxide and thorium hydroxide: these three adjacent hydroxides may be separated by precipitating them with ammonia.
Radium chloride is a colorless, luminescent compound. It becomes yellow after some time due to self-damage by the alpha radiation given off by radium when it decays. Small amounts of barium impurities give the compound a rose color. It is soluble in water, though less so than barium chloride, and its solubility decreases with increasing concentration of hydrochloric acid. Crystallization from aqueous solution gives the dihydrate RaCl2·2H2O, isomorphous with its barium analog.
Radium bromide is also a colorless, luminous compound. In water, it is more soluble than radium chloride. Like radium chloride, crystallization from aqueous solution gives the dihydrate RaBr2·2H2O, isomorphous with its barium analog. The ionizing radiation emitted by radium bromide excites nitrogen molecules in the air, making it glow. The alpha particles emitted by radium quickly gain two electrons to become neutral helium, which builds up inside and weakens radium bromide crystals. This effect sometimes causes the crystals to break or even explode.
Radium nitrate is a white compound that can be made by dissolving radium carbonate in nitric acid. As the concentration of nitric acid increases, the solubility of radium nitrate decreases, an important property for the chemical purification of radium.
Radium forms much the same insoluble salts as its lighter congener barium: it forms the insoluble sulfate, chromate, carbonate, iodate, tetrafluoroberyllate, and nitrate. With the exception of the carbonate, all of these are less soluble in water than the corresponding barium salts, but they are all isostructural to their barium counterparts. Additionally, radium phosphate, oxalate, and sulfite are probably also insoluble, as they coprecipitate with the corresponding insoluble barium salts. The great insolubility of radium sulfate means that it is one of the less biologically dangerous radium compounds. The large ionic radius of Ra results in weak ability to form coordination complexes and poor extraction of radium from aqueous solutions when not at high pH.