How many elements are in the actinide series?

The actinide series contains 15 elements, usually from actinium (89) to lawrencium (103). Many actinides are radioactive, and some do not occur naturally in meaningful amounts. Uranium and thorium are the best-known natural actinides, and both have major roles in nuclear science. Plutonium is another famous actinide, widely associated with nuclear reactors and weapons history. Because their electrons occupy the 5f subshell, actinides have complex chemistry and multiple oxidation states.