Actinider

Actinider is a term that in some languages or contexts appears as a spelling variant of actinide, the chemical family that comprises 15 metallic elements with atomic numbers from 89 (actinium) to 103 (lawrencium). In English usage the standard term is actinide, but actinider can be encountered in translations or historical texts. The group is named after actinium, the first element in the series, whose name ultimately derives from the Greek aktis or aktinos, meaning ray.

Actinides are f-block elements characterized by the progressive filling of the 5f electron orbitals. They are typically dense, soft metals with silvery appearance and a pronounced tendency toward radioactivity. Chemically, they exhibit a wide range of oxidation states, commonly from +3 to +6, and form complex compounds with a variety of ligands. The chemistry of actinides is intricate due to the comparable energies of their 5f, 6d, and 7s orbitals, leading to diverse bonding behavior and coordination environments.

Occurrence and production vary across the series. Several actinides occur naturally in trace amounts in uranium and thorium ores, while many others are synthetic, produced in nuclear reactors or particle accelerators. Notable members include uranium and plutonium, widely used in nuclear energy and weapons, as well as later actinides such as americium, curium, berkelium, californium, and others, which are important in research and specialized applications. Safety concerns center on radiation hazards, chemical toxicity, and environmental impact, requiring strict handling, containment, and regulatory controls.

See also: actinide series, nuclear chemistry, radioactive decay.