High Quality Content by WIKIPEDIA articles! Nuclear notation is a common notation used in chemistry and physics to indicate the atomic number and mass number of an isotope. Nuclear notation is formed by writing an elemental symbol preceded by a subscript indicating its atomic number (number of protons) and a superscript indicating its mass number (number of nucleons). For example, carbon-12 has an atomic number of 6 and a mass number of 12 and would thus be written as 126C. If the isotope is in a metastable state, an "m" can be added to the mass number. For example technetium-99m can be written as 99m43Tc. It is also common to drop the atomic number, as it is redundant with the symbol (12C, 99mTc).