ra·di·um (r??d?-?m)n. Symbol Ra A rare, brilliant white, luminescent, highly radioactive metallic element found in very small amounts in uranium ores, having more than 40 isotopes and isomers with mass numbers between 201 and 234, of which Ra-226 with a half-life of 1,600 years is the most common. It is used as a neutron source for some research purposes and was formerly widely used in cancer radiotherapy and as a constituent of luminescent paints. Atomic number 88; melting point 696°C; boiling point 1,737°C; specific gravity 5; valence 2. See Periodic Table.[Latin radius, ray; see ray1 + -ium.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.radium (?re?d??m) n (Elements & Compounds) a. a highly radioactive luminescent white element of the alkaline earth group of metals. It occurs in pitchblende, carnotite, and other uranium ores, and is used in radiotherapy and in luminous paints. Symbol: Ra; atomic no: 88; half-life of most stable isotope, 226Ra: 1620 years; valency: 2; relative density: 5; melting pt: 700°C; boiling pt: 1140°Cb. (as modifier): radium needle. [C20: from Latin radius ray]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014ra?di?um (?re? di ?m) n. a highly radioactive metallic element whose decay yields radon gas and alpha rays. Symbol: Ra; at. wt.: 226; at. no.: 88. [