The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.:20 The mass number is different for each isotope of a chemical element.
We write a mass number after an element's name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon has 6 protons and 6 neutrons. We write it as carbon-12 or 12C.
Mass number is not the same as: