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Protons and neutrons have nearly the same mass while electrons are much less massive. If we assume that a neutron has a mass of 1, then the relative masses are:
Neutron = 1
Proton = 0.99862349
Electron = 0.00054386734
Said another way, protons are only about 99.86% as massive as neutrons while electrons are only about 0.054% as massive as neutrons. While relative masses are nice if you want to compare protons, neutrons and electrons to one another, it doesn't tell you what the actual masses of these particles are. In kilograms, the masses are:
Neutron = 1.6749286*10-27 kg
Proton = 1.6726231*10-27 kg
Electron = 9.1093897*10-31 kg
There is another unit, called an electron volt (eV), that scientists use when talking about small things like protons, neutrons and electrons. An electron volt is actually a measurement of energy, but scientists can get away with using it to measure mass since mass and energy are related by Einstein's famous equation, E = mc2. So, in terms of MeV (Megaelectron volts, 1 MeV = 1,000,000 eV), the masses are:
Neutron = 939.56563 MeV
Proton = 938.27231 MeV
Electron = 0.51099906 MeV
Steve Gagnon, Science Education Specialist (Other answers by Steve Gagnon)
If different types of quarks have different masses, then why are protons and neutrons said to have the same mass?