Californium (Cf). Diagram of the nuclear composition and electron configuration of an atom of californium-251 (atomic number: 98), the most stable isotope of this element. The nucleus consists of 98 protons (red) and 153 neutrons (yellow). 98 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Californium, named for where it was first synthesised in 1950, is an actinide and transuranic element in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Californium-251 has a half-life of 898 years, its nucleus emitting an alpha particle to form curium-247.