The nuclear atom (spec 1.14, 1.15)
Tiny dense nucleus, electrons in shells, mostly empty space.
Atoms have a nuclear structure: a tiny dense nucleus at the centre containing protons and neutrons, with electrons moving in shells (energy levels) around the nucleus.
| Particle | Relative mass | Relative charge | Where it is |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1840 (negligible) | -1 | Shells around nucleus |
Mass. Almost all the mass of the atom is in the nucleus — electrons contribute negligibly because their relative mass is 1/1840. The protons and neutrons share the mass roughly equally.
Charge balance. In a NEUTRAL atom, the number of protons equals the number of electrons, so positive and negative charges cancel. An ion is formed when an atom GAINS or LOSES electrons:
- LOSE electrons → positive ion (cation), e.g. Na atom (11p, 11e) → Na⁺ (11p, 10e).
- GAIN electrons → negative ion (anion), e.g. Cl atom (17p, 17e) → Cl⁻ (17p, 18e).
Size. The nucleus is roughly 1/10,000 the diameter of the atom — most of the atom is empty space. This was first demonstrated by the Geiger–Marsden (Rutherford) alpha-scattering experiment: most α-particles passed straight through a thin gold foil (showing the atom is mostly empty), but ~1 in 8000 were deflected backwards (showing a tiny, dense, positive nucleus).
Mark-scheme tip. Edexcel mark schemes credit "electrons in shells", "nucleus contains protons + neutrons", and "most of the atom is empty space" as distinct mark-bearing keywords. Use all three.
- Nucleus = protons + neutrons; tiny but contains essentially all the mass.
- Electrons orbit in shells; mass negligible.
- Most of the atom is empty space.
- Ions form by gain (anion) or loss (cation) of electrons.