Inside the atom: nucleus, protons, neutrons, electrons
Ernest Rutherford, a physicist born in New Zealand (1871–1937), did experiments in the early 1900s that showed the atom has a tiny, dense, positive centre: the nucleus. He won the Nobel Prize in Chemistry in 1908. The nucleus holds protons (charge +1) and neutrons (charge 0). Electrons (charge −1) move around the nucleus. A proton and a neutron each have a mass of about 1 u, a tiny unit made for atoms, while an electron has only about 1/1836 u. So almost all the mass of an atom is in its nucleus. The nucleus is also very small: if the atom were a huge sports field, the nucleus would be like a pinhead in the middle, and the electrons would move around in the space of the whole field. An atom is neutral because it has equal numbers of positive protons and negative electrons.
| Particle | Where | Charge | Mass |
|---|---|---|---|
| proton | nucleus | +1 | 1 u |
| neutron | nucleus | 0 | 1 u |
| electron | around the nucleus | −1 | 1/1836 u |
Charge cards. Make 8 small cards marked + (protons) and 8 marked − (electrons). A neutral atom needs the same number of + cards and − cards. Choose 5 + cards: how many − cards make the atom neutral? Try it again with 8 + cards. Then take away one − card and say what the total charge is.