The subatomic particles that contribute most to an atom’s mass are protons and neutrons. Together, they are found in the nucleus of the atom and are significantly more massive than electrons.
Protons carry a positive charge and have a mass of approximately 1 atomic mass unit (amu). Neutrons, which have no charge, also have a mass close to 1 amu. In contrast, electrons are much lighter, with a mass of about 1/1836 of an amu. Because of this mass difference, the majority of an atom’s mass comes from the protons and neutrons in the nucleus.
When considering an element’s atomic mass, we primarily look at the number of protons and neutrons, since the contribution of electrons is negligible due to their much smaller mass. For example, a carbon atom has 6 protons and 6 neutrons, giving it a mass that is roughly 12 amu, with electrons contributing inconsequentially to this total.