Ununpentium (Uup). Diagram of the nuclear composition and electron configuration of an atom of ununpentium-288 (atomic number: 115), the most stable isotope of this radioactive element. The nucleus consists of 115 protons (red) and 173 neutrons (yellow). 115 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Ununpentium is a temporary name (2003 synthesis not yet official). It is a transactinide of unknown properties in group 15, period 7, and the p-block of the periodic table. The ununpentium-288 nucleus (half-life: 173 milliseconds) emits an alpha particle to form ununtrium-284.