mass number

An element isotope's mass number (or atomic mass number or nucleon number, typically indicated by symbol A) is the number of nucleons (protons and neutrons) in its nucleus, the element's particular isotope being defined as those with nuclei that have this particular mass number. It approximates an atom's or nucleus's mass in units of the mass of a nucleon, but this is not exact because the mass of a proton differs slightly from the mass of a neutron, and also, different isotopes incorporate different amounts of binding energy to hold the nuclei together. Also, for an atom, due to its electrons. The mass number can be indicated by a superscript to the left of the element's symbol, e.g., ¹²C for the carbon (C) isotope with mass number 12.