Mass number

The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus. 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:
The difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: "N "= "A "− "Z".
The mass number is not shown on the periodic table. For each element, what is shown is the "standard atomic weight" and the "atomic number".