Plutonium Plutonium is a radioactive metallic element. Although it is occasionally found in nature, mostly all of our plutonium is produced artificially in a lab. The official chemical symbol for plutonium is Pu, coming from its first and third letters. Its atomic number is ninety-four. Plutonium is able to maintain its solid state until very high temperatures, melting at six hundred and forty degrees Celsius, and boiling at three thousand four hundred and sixty degrees. The density of Plutonium, at twenty degrees centigrade, is 19.86 grams per cubic centimeter. Plutonium was discovered, in the laboratory, by Glenn Theodore Seaborg,