# What is the nucleon number?

Jun 3, 2014

The mass number (A), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element.

Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: N=A−Z