Which particle is responsible for the mass of an atom?

The mass of an atom primarily comes from the protons and neutrons found in its nucleus. Both of these particles are nucleons, and they contribute significantly to the atomic mass due to their relative mass compared to electrons, which are much lighter and negligible in terms of mass contribution. Protons have a positive charge and neutrons are neutral, but both have similar mass, which is about 1 atomic mass unit (amu).

Electrons, while they contribute to the overall atom's structure by influencing chemical properties and the atom's behavior in reactions, are approximately 1/1836 the mass of a proton or neutron. Therefore, their mass contribution is effectively ignored when calculating the atomic mass. Thus, the correct understanding is that the combined mass of neutrons and protons directly accounts for the majority of an atom's mass, validating that both particles are responsible for this property of the atom.