Plutonium (Pu). Diagram of the nuclear composition and electron configuration of an atom of plutonium-239 (atomic number: 94), an isotope of this element. The nucleus consists of 94 protons (red) and 145 neutrons (yellow). 94 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Plutonium is an actinide and transuranic element in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. The nucleus of plutonium-244 (half-life: 80 million years) emits an alpha particle to form uranium-240. Other plutonium isotopes are used in nuclear fuels and weapons.