Plutonium is made through the fission, or splitting, of uranium atoms. During this process, some uranium-238 isotopes gain a neutron given off by the split atoms, forming new uranium-239 isotopes that eventually turn into plutonium-239 as they decay.
Plutonium was first discovered by four American chemists at the University of California Berkeley in 1941. However, due to World War II, the U.S. government forced them to keep the discovery a secret until 1948. Although plutonium is generally considered a man-made element that can typically only be created in a nuclear reactor, extremely small amounts of plutonium have been found to occur in nature under certain extreme and rare geologic conditions.
There are 15 known plutonium isotopes, each of which has a different number of neutrons and thus a different atomic mass. All 15 isotopes are highly radioactive as they quickly decay, causing them to give off particle radiation. The atomic number of plutonium is 94, meaning this is the number of electrons it has.
Although plutonium is a by-product of uranium fission, in nuclear power plants, some of the plutonium that is created also burns along with the uranium and creates electricity. Nearly one-third of the electricity created in a nuclear power plant comes from plutonium.