Do All Protons Have the Same Mass?

Protons all have the same mass; each proton's mass is 938.27 megaelectronvolts, or 1.6726231*10-27 kilograms. In fact, all protons are identical, as are all electrons and all neutrons, though each of these three subatomic particles is distinct from the other two kinds. Protons and neutrons have nearly the same mass, with protons weighing only slightly less than neutrons (1.6749286*10-27 kg), but electrons are lighter than both protons and neutrons, weighing in at 9.1093897*10-31 kg.

Protons, neutrons and electrons are subatomic particles that make up all atoms. Atoms are the building blocks of everything in the universe. The protons and neutrons make up the nucleus of the atom; protons and neutrons provide the bulk of an atom's weight, so the majority of the mass of an atom is in the nucleus, with the thin outer electron cloud providing comparatively little weight.

In a different unit of mass, a proton is 1.007316 atomic mass units, while a neutron is 1.008701 atomic mass units. Because Einstein proved that mass and energy are the same (E = mc2), it is possible for scientists to use either or both energy units and mass units to measure a proton's mass.