actinides

The actinides are a series of 15 metallic elements in the periodic table, with atomic numbers 89 through 103. They lie in the f-block and are defined by filling the 5f subshell. The series is named after actinium, the first element in the set. All actinides are radioactive; most are synthetic, produced in nuclear reactors or particle accelerators, but thorium and uranium occur naturally in the Earth's crust in small quantities, and some appear as decay products of heavier elements.

Chemically, actinides share some similarities with the lanthanides but show greater variability in oxidation states and bonding, due in part to the involvement of 5f electrons. This yields a wide range of compounds and coordination chemistries. Oxidation states commonly range from +3 to +6, with higher or lower states observed in individual elements.

Physically, actinides are dense, soft metals that are often silvery when freshly prepared but tarnish in air. They are reactive with air and moisture to varying degrees and, for several members, pyrophoric. As radioactive materials, they require specialized handling, containment, and shielding.

Natural occurrence and production: thorium and uranium occur naturally in the crust; most other actinides are synthetic, created in reactors or accelerators. Some appear as trace decay products.

Uses and significance: uranium and plutonium have central roles in nuclear power and weapons. Americium is used in smoke detectors. Other actinides are largely confined to research, isotope production, or specialty applications.