If protons and neutrons each weighed exactly 1 amu, and there was no change in mass when the nucleus was formed, then the mass number of an isotope would equal the sum of the masses of the protons and neutrons in amu, or its atomic weight. This is not strictly true. Not only are protons and neutrons slightly heavier than 1 amu, there is a small loss in mass when they combine to form a nucleus.

This missing mass is converted to energy during the nucleus-forming process and is lost by the atom. The nucleus cannot be taken apart again unless the lost energy is resupplied to make up the full mass, that is, the mass of the initial protons and neutrons. This missing energy represents the binding energy of the nucleus, or the energy that holds the nucleus together.

Nevertheless, for approximate calculations we can think of the atomic weight of an isotope as being approximately equal to the sum of its protons and neutrons, or to its mass number.