What does the term 'atomic mass' exclude in its calculation?

Atomic mass refers to the average mass of an atom of an element, taking into consideration the abundance of its isotopes. It is calculated by weighing the masses of all the isotopes of an element based on their natural abundance.

In this context, the atomic mass is primarily influenced by the number of protons and neutrons in the nucleus of an atom, as these are the main contributors to the mass. Electrons, on the other hand, have a significantly smaller mass compared to protons and neutrons and do not significantly contribute to the atomic mass. Therefore, the calculation of atomic mass effectively excludes the contribution of the number of electrons. This distinction is crucial because, while protons and neutrons are nearly equal in mass and contribute substantially to the overall atomic mass, electrons are negligible in this regard.