Final answer:
Electrons have an extremely small mass compared to protons and neutrons. Most of the mass of an atom comes from the nucleus, which contains neutrons and protons.
Step-by-step explanation:
Even though electrons, protons, and neutrons are all types of subatomic particles, electrons have an extremely small mass, compared to either protons or neutrons. On the other hand, the masses of protons and neutrons are fairly similar. Because protons and neutrons are so much more massive than electrons, almost all of the mass of any atom comes from the nucleus, which contains all of the neutrons and protons.