Radium was discovered (1898) by Pierre Curie, Marie Curie (Curie, Marie), and an assistant, G. Bémont, after Marie Curie had observed that the radioactivity of pitchblende was four or five times greater than that of the uranium it contained and not fully explained on the basis of radioactive polonium, which she had just discovered in pitchblende residues. The new, powerfully radioactive substance could be concentrated with barium, but because its chloride was slightly more insoluble it could be concentrated by fractional crystallization. The separation was followed by the increase in intensity of new lines in the ultraviolet spectrum and by a steady increase in the apparent atomic weight of the material until a value of 225.18 was obtained, remarkably close to the accepted value of 226.03. By 1902, 0.1 gram of pure radium chloride was prepared by refining several tons of pitchblende residues, and by 1910 Marie Curie and André-Louis Debierne had isolated the metal itself.

Thirty-three isotopes of radium, all radioactive, are known; their half-lives, except for radium-226 (1,600 years) and radium-228 (5.8 years), are less than a few weeks. The long-lived radium-226 is found in nature as a result of its continuous formation from uranium-238 decay. Radium thus occurs in all uranium ores, but it is more widely distributed because it forms water-soluble compounds; the Earth's surface contains an estimated 1.8 × 10¹³ grams of radium.

Since all the isotopes of radium are radioactive and short-lived on the geological time scale, any primeval radium would have disappeared long ago. Therefore, radium occurs naturally only as a disintegration product in the three natural radioactive-decay series (thorium, uranium, and actinium series). Radium-226 is a member of the uranium-decay series. Its parent is thorium-230 and its daughter radon-222. The further decay products, formerly called radium A, B, C, C′, C″, D, etc., are isotopes of polonium, lead, bismuth, and thallium.

The chemistry of radium is what would be expected of the heaviest of the alkaline earths, but the intense radioactivity is its most characteristic property. One gram of radium-226 undergoes 3.7 × 10¹⁰ disintegrations per second, producing energy equivalent to 6.8 × 10⁻³ calories, sufficient to raise the temperature of a well-insulated sample at the rate of 1° C every 10 seconds. The practical energy release is even greater than this due to the production of a large number of short-lived radioactive decay products. The alpha particles emitted by radium may be used to initiate nuclear reactions.

Radium's uses all result from its radiations. The most important use of radium was formerly in medicine, principally for the treatment of cancer by subjecting tumours to the gamma radiation of its daughter isotopes. In many therapeutic applications radium has been superseded by the less costly and more powerful artificial radioisotopes cobalt-60 and cesium-137. An intimate mixture of radium and beryllium is a moderately intense source of neutrons, used for scientific research and for well logging in geophysical prospecting for petroleum. For these uses, however, substitutes have become available. One of the products of radium decay is radon, the heaviest noble gas; this decay process is the chief source of that element.

When concentrated, radium glows in the dark. Because of this property, it was once mixed with a paste of zinc sulfide, which could be excited by alpha particles, to make a self-luminescent paint for watch, clock, and instrument dials. During the 1930s it was found, however, that exposure to radium posed a serious hazard to health: a number of women who worked with the radium-containing luminescent paint during the 1910s and '20s had subsequently died. They had ingested considerable amounts of radium through the habit of licking the points of their brushes. Radium tends to concentrate in bone where alpha radiation interferes with red corpuscle production, so some of these women developed anemia and bone cancer. The practice of employing radium in luminescent coatings was halted in the early 1950s after the high toxicity of the material was recognized. Less hazardous alpha emitters have largely replaced radium. (The detection of exhaled radon provides a very sensitive test for radium absorption.)

In the thorium-decay series of radioactive elements, two radium isotopes occur. They are found naturally in the mineral monazite: radium-228 (6.7-year half-life) and radium-224 (3.64-day half-life). One of their descendants, thallium-208, emits gamma radiation even more penetrating than that of bismuth-214; and, as a result of the complex sequence of half-lives, the gamma activity of freshly purified radium-228 increases for about four years and then steadily decreases. A fourth isotope, radium-223 (11.7-day half-life), occurs in the actinium-decay series.

Metallic radium has high chemical reactivity. It is attacked by water with vigorous evolution of hydrogen and by air with the formation of the nitride. It occurs exclusively as the Ra²⁺ ion in all its compounds. The sulfate, RaSO₄, is the most insoluble sulfate known, and the hydroxide, Ra(OH)₂, is the most soluble of the alkaline-earth hydroxides. Its compounds are very similar to the corresponding barium compounds, making separation of the two elements difficult.

In modern technology, radium is separated from barium by fractional crystallization of the bromides, followed by purification by ion-exchange techniques for removal of the last 10 percent of the barium. Radium metal may be prepared by electrolytic reduction of its salts.