ACTINIDE

any of a series of 15 consecutive chemical elements in the periodic table from actinium to lawrencium (atomic numbers 89-103). All the actinide elements are heavy metals that are unstable toward radioactive decay. Only four of them-actinium, thorium, protactinium, and uranium-occur in nature in appreciable quantities. The remaining 11 members from neptunium to lawrencium, which are frequently classified as transuranium elements, are produced artificially by bombardment of other related elements with high-energy particles. Uranium (or more properly its isotope uranium-235) is by far the most important actinide because of its fissionability-that is, the tendency of its nucleus to split into fragments of almost equal mass. The result of this fission is the release of large amounts of energy. Both uranium and plutonium are used in nuclear power plants for the production of electrical power and in nuclear bombs for their explosive power. The actinide elements have a similar electronic structure, which is reflected by strong affinities in their chemical properties and behaviour. Actinides generally show several oxidation states. (When an element combines with others to form a compound, the number of electrons of its atoms that can be involved in the bonds of that particular compound is called the oxidation state of the element.) Most common among the actinides are the +3 and +4 oxidation states. The actinides in both these states bear a distinct resemblance to members of the lanthanide series in identical states. The lanthanides consist of the rare-earth elements with atomic numbers 57-71.