actinide

Actinide is a term used in chemistry to describe a series of 15 metallic elements in the periodic table, with atomic numbers from 89 to 103. The actinide series forms the bottom row of the f-block and is named after actinium, the first member. The elements in the series are actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.

Most actinides are radioactive, and natural abundances are extremely limited outside of uranium and thorium. Several actinides occur in trace amounts in uranium- and thorium-rich minerals, while others are produced synthetically in nuclear reactors or particle accelerators. They are generally dense, malleable metals that can be slippery and tarnish in air. Many actinides are pyrophoric or require special handling due to radiological hazards.

Chemically, actinides share features of the f-block elements. In aqueous solutions, the common oxidation state is +3, but several actinides exhibit a range of higher oxidation states (up to +4, and for some elements even +5, +6, or beyond) depending on conditions. The chemistry is influenced by the behavior of 5f electrons and relativistic effects, which leads to complex bonding patterns and a tendency for variable coordination.

Uses and significance include nuclear energy and research. Uranium and thorium serve as sources of nuclear fuel; plutonium, americium, and curium are produced in reactors and have specialized applications, including radiological power sources and medical or scientific uses. Because actinides are highly radioactive and chemically toxic, they require stringent safety and environmental controls.