 # ⓘ Mass number. The mass number 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 iso .. ## ⓘ Mass number

The mass number 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 elements name or as a superscript to the left of an elements symbol. For example, the most common isotope of carbon has 6 protons and 6 neutrons. We write it as carbon-12 or 12 C.

Mass number is not the same as:

• atomic mass symbol: m a, which is the mass of a single atom, commonly expressed in unified atomic mass units
• relative atomic mass symbol: A r, also called atomic weight, which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.
• atomic number symbol: Z, which is the number of protons in a nucleus of an atom

The difference between the mass number and the atomic number gives the number of neutrons 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.