ACTINIUM AND THE ACTINIDES

Following actinium (group 3) are the 14 components of the actinide series (represented by the symbol An) combined with progressive filling of the 5f shell and so analogous to the lanthanides. All are radioactive; their largest-lived isotopes being shown in the progressively shorter half-lives reflect the decreasing stability of heavy nuclei, producing from the changing balance between the attractive strong communication and the repulsive Coulomb forces. Most actinide nuclei undergo α decay by emitting 4He, but for heavier components spontaneous fission into two fragments is an increasingly important alternative decay route.

Only uranium and thorium have half-lives long enough to survive since the formation of the Earth. Thorium is found together with lanthanides in the phosphate mineral monazite (LnPO4), and uranium perform as arnotite K2(UO2)2(VO4)2.3H2O and pitchblende U3O8c. Uranium is principally need as a nuclear fuel, as the isotope 235U undergoes neutron-induced fission, the nucleus divided into two smaller fragments together with more neutrons, which may thus initiate a chain reaction. The energy liberated (about 2×1010 kJ mol-1) is vastly larger than that obtainable from chemical reactions.