A chemical element, Ra, with atomic number 88. The atomic weight of the most abundant naturally occurring isotope is 226. Radium is a rare radioactive element found in uranium minerals to the extent of 1 part for about every 3 × 10⁶ parts of uranium. Chemically, radium is an alkaline-earth metal having properties quite similar to those of barium. Radium is important because of its radioactive properties and is used primarily in medicine for the treatment of cancer, in atomic energy technology for the preparation of standard sources of radiation, as a source for actinium and protactinium by neutron bombardment, and in certain metallurgical and mining industries for preparing gamma-ray radiographs.