3 Particles
Inside the Nucleus:
Protons = +1 relative charge, relative mass of 1.
Neutrons = 0 relative charge, relative mass of 1.
Outside of Nucleus (On on shells):
Electrons = -1 relative charge, relative mass of 0.0005.
Atoms are neutral since there is always the same number of protons as electrons, so the charges cancel each other out.
