Protons and neutrons account for most of an atom's mass. These subatomic particles are found in the nucleus of the atom, where they form nucleons.
Protons and neutrons have very similar atomic masses. Neutrons have an atomic mass of 1.0078 amu while protons have an atomic mass of 1.0073 amu, according to the UC Davis ChemWiki. Electrons are the lightest subatomic particles. Unlike protons and neutrons, electrons circle the nucleus in orbitals. Electrons have an atomic mass of 0.00054858 amu. Neutrons do not affect the overall charge of an atom, but they do play a role in the formation of isotopes, which are atoms of the same element that have different numbers of neutrons.