Actinide Elements, or Actinides, a series of 15 chemical elements (atomic numbers 89 through 103). The series is named after actinium, the first element of the group. The other actinides, in order of increasing atomic number, are thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium. The actinides with atomic numbers higher than 92 (neptunium through lawrencium) are also called transuranium elements.

All the actinides are radioactive metals. Because of the arrangement of their orbital electrons, they are chemically similar to one another and also to the lanthanides, or rare-earth metals.

Thorium, protactinium, uranium, and very small amounts of actinium and plutonium are found in nature. The other actinides, along with most plutonium, are produced artificially in nuclear reactors and particle accelerators. Uranium, plutonium, and thorium are used to produce nuclear energy. Thorium is also used as an alloying agent. Other actinides have applications in nuclear research, medicine, and industry.