In nuclear physics and nuclear chemistry, the various species of atoms whose nuclei contain particular numbers of protons and neutrons are called nuclides. Nuclides are also characterized by their nuclear energy states (e.g., metastable nuclide 242mAm). Each nuclide is denoted by the chemical symbol of the element (this specifies Z) with the atomic mass number as a superscript. Hydrogen (H), for example, consists of one electron and one proton. The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The total number of nucleons, protons, and neutrons in a nucleus are equal to Z + N = A, where A is called the atomic mass number.
Isomers are nuclides with equal proton number and equal mass number (thus making them by definition the same isotope) but different energy states. We usually indicate isomers with a superscript m, thus: 241mAm or 110mAg.
Isomers are usually associated with gamma decay. Gamma rays are emitted by unstable nuclei in their transition from a high-energy state to a lower state. This transition is known as the isomeric transition. The emission of a gamma-ray from an excited nuclear state allows the nucleus to lose energy and reach a lower energy state, sometimes its ground state. In certain cases, the excited nuclear state following a nuclear reaction or other radioactive decay can become a metastable nuclear-excited state. Some nuclei can stay in a metastable state for a long time (hours, days, and sometimes much longer). The long-lived excited nuclei are known as isomeric states (or isomers) for minutes, hours, days, or occasionally far longer before undergoing gamma decay, in which they emit a gamma-ray. Extremely unstable nuclei that decay as soon as they are formed in nuclear reactions (half-life less than 10-11s) are not generally classified as nuclear isomers.