Thorium
. Thorium, symbol Th, radioactive metallic element with an atomic number of 90. Thorium is a member of the actinide series of the periodic table . Thorium was discovered (1828) by the Swedish chemist Baron Jöns Jakob Berzelius . The element is dark in color, slowly attacked by water, soluble in hydrochloric and sulfuric acids, and slightly soluble in nitric acid. It ranks 39th in abundance among the elements in the crust of the earth. Thorium melts at about 1750° C (about 3182° F), boils at about 4850° C (about 8762° F), and has a specific gravity of 11.8. The atomic weight of thorium is 232.04. Small quantities of thorium are found in thorite, or thorium silicate; in orangite, a variety of thorite; and in thorianite, a radioactive mineral composed of thorium oxide and uranium. Thorium has isotopes ranging in mass number from 212 to 236. Thorium-232 occurs naturally, has a half-life of about 14 billion years, and is the first member of the radioactive-decay series, ending with the sta