Plutonium-239 is the main radioactive isotope used in nuclear bombs. It is also used as a nuclear reactor fuel.
It has 94 protons, and has a mass on average of about 244.
Plutonium has a MASSIVE half life, so releases it's radiation very slowly. Plutonium-239 minly releases alpha particles, but can also release beta and gamma.
Plutonium is the main reason why the land around Chernobyl is still uninhabitable.