Britannica Sources used in this answer

How is mass number calculated for an atom?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Bohr model diagram of a hydrogen atom showing a green nucleus labeled H, a single electron orbit, and text with atomic mass 1.008 and electron configuration 1. — atom The Bohr model describes the structure of atoms, in this case hydrogen.

The mass number of an atom is calculated by finding the sum of the number of protons and neutrons present in its nucleus.12

These two types of particles, collectively known as nucleons, account for virtually all of an atom's mass. Because the number of protons defines the atomic number of an element, the mass number is primarily used to distinguish between different isotopes of the same element, which share the same number of protons but contain varying numbers of neutrons.213