* Protons and neutrons are the two main subatomic particles found in the nucleus. They are much heavier than electrons, which orbit the nucleus.
* Electrons contribute very little to the overall mass of an atom. Their mass is about 1/1836 that of a proton or neutron.
* Atomic mass is a weighted average of the masses of all the isotopes of an element. Isotopes have the same number of protons but different numbers of neutrons, which affects their mass.
Therefore, the number of protons and neutrons in an atom, which make up the vast majority of its mass, determine its overall mass.
