Thorium (Th). Diagram of the nuclear composition and electron configuration of an atom of thorium-232 (atomic number: 90), the most stable isotope of this element. The nucleus consists of 90 protons (red) and 142 neutrons (yellow). 90 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Thorium is an actinide in period 7, and the f-block of the periodic table. Actinides are soft radioactive metals. Thorium-232, which is used as a nuclear fuel, contributes a large part of the Earth's internal heat. It has a half-life of 14 billion years. Its nucleus decays by emitting an alpha particle to form radium-228.