An atom consists of a nucleus which contains two types of subatomic particles - protons and neutrons. Electrons - a further type of subatomic particle - occupy a region in space outside the nucleus and are arranged in
shells.
The mass of a proton is about 1.67 x 10
-27kg. The charge of a proton is +1.60217733 x 10
-19C. Comparing the masses and charges of protons, neutrons and electrons is more convenient using relative masses and charges.
| relative mass | relative charge |
| proton | 1 | +1 |
| neutron | 1 | 0 |
| electron | 1/1836 | -1 |
