Actinium

Actinium, symbol Ac, radioactive metallic element found in all uranium ores. The atomic number of actinium is 89; the element is in the actinide series of the periodic table.

Actinium was discovered in 1899 by the French chemist André Louis Debierne. The element is found in uranium ores to the extent of 2 parts to every 10 billion parts of uranium. Two naturally occurring isotopes of actinium are known. Actinium-227 is a member of the actinium series, called the actinium decay series, resulting from the radioactive decay of uranium-235. It has a half-life of 21.8 years. The other isotope, actinium-228, is a member of the thorium series resulting from the decay of thorium-232. This isotope, known also as mesothorium-2, has a half-life of 6.13 hours. Isotopes ranging in mass number from 209 to 234 are known. Actinium melts at about 1050° C (1922° F), boils at about 3200° C (5792° F), and has a relative density of about 10.