(Lat. radius, “ray”), radioactive metallic element, symbol Ra, one of the ALKALINE EARTH METALS, (q.v.) in group 2 (or IIa) of the periodic table (see PERIODIC LAW,); at.no. 88, at.wt. (most stable isotope) 226. Radium-226 metal melts at about 700° C (about 1292° F), and has a sp.gr. of 5.5.

Radium was discovered in the ore pitchblende by the French chemists Marie Curie and Pierre Curie in 1898. They discovered that the ore was more radioactive than its principal component, URANIUM, (q.v.), and they separated the ore into many chemical fractions in order to isolate the unknown sources of RADIOACTIVITY, (q.v.). One fraction, isolated by use of bismuth sulfide, contained a strongly radioactive substance that the Curies showed was a new element, POLONIUM, (q.v.). A highly radioactive barium-chloride fraction was treated to remove the radioactive substance, which was discovered to be a new element, radium.

Properties and Occurrence.

Radium is a silver-white metal that oxidizes immediately upon exposure to air. The element is used and handled in the form of radium chloride or radium bromide and practically never in the metallic state.

Radium is formed by the radioactive disintegration of uranium and is consequently found in all uranium ores. Radium is present in uranium ore to the extent of 1 part of radium to 3 million of uranium. It is extracted from the ore by the addition of a compound of barium that acts as a “carrier.” The chemical properties of radium are similar to those of barium, and the two substances are removed from the other components of the ore by precipitation of barium and radium sulfate. The sulfates are converted into carbonates or sulfides, which are then dissolved in hydrochloric acid. The radium is separated from the barium as the end result of successive crystallizations of the chloride solutions.

Of the isotopes (see ISOTOPE,) of radium, ranging in mass number from 206 to 232, the most abundant is the isotope of mass 226. Radium-226 is formed by the radioactive disintegration of the thorium isotope of mass 230, which is the fourth successive isotope in the decay series starting with uranium-238. The half-life of radium-226 is 1620 years. It emits alpha particles, forming the gas RADON, (q.v.).

Uses.

Radiation from radium has a harmful effect upon living cells, and radium burns are caused by overexposure to the rays. Cancerous cells, however, are often more sensitive to radiation than normal cells, and such cells may be killed without seriously injuring healthy tissue by controlling the intensity and direction of the radiation. Radium is now used in the treatment of only a few kinds of cancer; radium chloride or radium bromide is enclosed in a sealed tube and inserted in the diseased tissue. When a radium salt is mixed with a substance such as zinc sulfide, the substance is caused to luminesce by the bombardment of the alpha rays emitted by the radium. Small amounts of radium were once used in the production of luminous paint, and applied to clock dials, doorknobs, and other objects, to make them glow in the dark.