质量数（Mass number）是指中性原子中，原子核内 质子数目和中子数目的总和，质量数的数值都是整数。

如氧，其中性原子的原子核内质子数和中子数皆为8，故其质量数为16与其实际原子量15.9994 极为接近。

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. It determines the atomic mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. 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.