Inside the atom
The building unit of all matter is the atom.
An atom is not the smallest thing there is. It is itself made of smaller pieces — subatomic particles — and three of them matter here: the proton, the electron and the neutron.
| Particle | Charge | Mass |
|---|---|---|
| Proton | Positive | 1 unit |
| Neutron | None — neutral | About the same as a proton |
| Electron | Negative | About 1/1840 of a proton — near enough to nothing |
Why an atom carries no overall charge
An atom holds as many electrons as protons. Their charges are equal and opposite, and the neutron adds none at all — so the positive and the negative cancel exactly, and the atom is electrically neutral.
Keep the two facts in that table apart, because each is about to do different work. Charge is what decides how the particles are arranged and how atoms bond. Mass is what decides how heavy an atom is — and since the electron has almost none, the mass of an atom is settled by its protons and neutrons alone.