A nuclide is a type of atom characterized by its proton number, neutron number and its energy condition. Conditions with a life of less than 10-10s are called excited conditions of a nuclide. At present, approximately 4 000 experimentally observed nuclides and isomers are known, distributed over the 118 currently known elements. More than 3 700 nuclides of these are radioactive. (See ‘radionuclides’).