The mass number (symbol: A) of an atom is the sum of the number of protons and the number of neutrons in the nucleus.^:20 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:

• atomic number (symbol: Z), which is the number of protons in a nucleus of an atom
• 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.