Fermium (Fm). Diagram of the nuclear composition and electron configuration of an atom of fermium-257 (atomic number: 100), this element's most stable isotope. The nucleus consists of 100 protons (red) and 157 neutrons (yellow). 100 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Fermium, named after Enrico Fermi and first synthesised in 1952, 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 fermium-257 (half-life: 100 days) emits an alpha particle to form californium-253.