Symbol Nh. A radioactive transuranic element that is a p-block transactinide element; a.n. 113; mass number of the most stable isotope 286, with a half-life of about eight seconds. It has been predicted that the heavier isotope rihonium-287 might have a half-life of about 20 minutes, but this isotope has not been discovered yet. There are seven main isotopes of nihonium. Nihonium was discovered in the 1900s using a nuclear reaction between bismuth-209 and zinc-70.