Meitnerium (Mt). Diagram of the nuclear composition and electron configuration of an atom of meitnerium-278 (atomic number: 109), the most stable isotope of this radioactive element. The nucleus consists of 109 protons (red) and 169 neutrons (yellow). 109 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). Meitnerium, named after Lise Meitner and first synthesised in 1982, is a transactinide of unknown properties in group 9, period 7, and the d-block of the periodic table. Meitnerium-278 has a half-life of 7.6 seconds, its nucleus emitting an alpha particle to form bohrium-274.