A nuclide is an atom of a specific isotope, characterized by its atomic number, mass number, and its energy state. Conditions with a life of less than 10^-10 sec are called excited conditions of a nuclide. At present, more than 2,770 different nuclides are known, distributed over the 113 currently known elements. More than 2,510 nuclides of these are radioactive (see radionuclide).