What subatomic particles make up the majority of an atom's mass?

The majority of an atom's mass comes from neutrons and protons. These subatomic particles are located in the nucleus of the atom, where nearly all of the atom's mass is concentrated. Neutrons have a mass similar to protons, which are significantly larger than that of electrons. While electrons do contribute to the overall mass of an atom, their contribution is negligible compared to that of protons and neutrons.

Protons carry a positive charge and define the atomic number of an element, while neutrons are neutral and add mass without affecting the charge. Together, these particles account for approximately 99.9% of an atom's total mass. The presence of electrons, although crucial for chemical reactivity and bonding, plays a minor role in the overall mass when evaluating an atom.

Thus, the combination of neutrons and protons provides the correct understanding of where an atom's mass predominantly resides.