Plutonium is a metallic chemical element classified among the actinides on the periodic table of elements. This highly radioactive element is used primarily in weapons and nuclear power plants, and it has become somewhat well known because of these uses. In nature, plutonium is relatively rare, occurring in uranium rich ores in trace amounts; most of the world's working supply of this element is obtained through neutron bombardment of uranium, a close neighbor on the periodic table.

When plutonium is isolated, its appearance may vary, because it has six allotropic forms, meaning that the element has six different structures under normal conditions. These forms vary in terms of density, although they all share the basic chemical properties of toxicity, radioactivity, and reactivity with many other elements. Most forms of plutonium are silvery-gray, but they oxidize to a dull yellow over time. Quantities which are large enough are also warm to the touch, due to the alpha particles they emit as they age.

On the periodic table of elements, plutonium is identified with the symbol Pu and the atomic number 94. Credit for the discovery of the element is usually given to a team of scientists at the University of California, Berkeley, who uncovered it in 1940 while studying the properties of uranium. Gleen T. Seaborg led the team, which also included Edwin McMillan, Joseph Kennedy, and Arthur Wahl. Initially, the discovery of the element was kept secret, due to concerns about its potential military applications.

This element is named for the planet Pluto, in a reference to the preceding elements of the periodic table, neptunium and uranium. In addition to being used in weapons and power plants, plutonium also appears in medical research and nuclear research. It can be difficult to work with, as it can exhibit strange behaviors, sometimes in response to very small fluctuations in its conditions.

Like other radioactive elements, plutonium represents a health risk. It can accumulate in bone marrow and organs such as the liver, causing damage as it emits radiation. The element can also form compounds which will spontaneously combust at room room temperature, and it can react alarmingly with some other elements. Because plutonium is so rare, average consumers will rarely, if ever interact with it, and people who utilize the element in their work are usually given meticulous safety training.