Mass Number — Definition & Overview

The mass number is really also 12.01, it is just convention to round it to a whole number.

The mass number is the sum of the protons and neutrons in each atom, and the atomic mass is calculated based on the average mass per atom for many atoms, which sometimes have a different amount of neutrons than the average.

Neutrons and protons are not the same mass and therefore just counting the sum of them in an atom won't find the exact mass of the atom.

The mass number is the sum of the protons and neutrons in each atom, and the atomic mass is calculated based on the average mass per atom for many atoms, which sometimes have a different mass per neutron than the average.