The structure of an atom
An atom has a tiny, dense nucleus containing protons (positive) and neutrons (neutral), surrounded by a cloud of electrons (negative) in shells/orbits.
Sizes and scale
- Atom diameter: ~1 × 10⁻¹⁰ m (= 0.1 nm).
- Nucleus diameter: ~1 × 10⁻¹⁴ m.
The nucleus is 10 000 times smaller than the atom — most of the atom is empty space. If an atom were the size of a football stadium, the nucleus would be a pea on the centre spot.
Particles and charges
| Particle | Charge | Relative mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1/1836 ≈ 0 |
- The nucleus carries virtually all the mass.
- Electrons contribute negligibly to mass but determine chemistry.
Atomic vs nuclear language
- Atomic number Z = number of protons (also = electrons in a neutral atom).
- Mass number A = protons + neutrons.
- Number of neutrons = A − Z.
Standard notation: $^A_Z X$.
Electron shells
Electrons sit in energy levels (or "shells") around the nucleus. The lowest level fills first; outer shells are higher in energy.
If an atom absorbs a photon, an electron may jump to a higher shell. When it falls back, it emits a photon of the same energy difference. Different atoms have different energy gaps, giving each element a unique emission spectrum.
Ions
If an atom loses an electron, it becomes a positive ion (cation). If it gains one, a negative ion (anion). Ionising radiation (alpha, beta, gamma) creates ions by knocking electrons off atoms.
⚠Common mistakes
- Saying the nucleus contains electrons — it doesn't.
- Confusing Z (protons) and A (protons + neutrons).
- Forgetting that the nucleus has nearly all the mass.
- Drawing electrons in fixed circular orbits — modern QM gives "orbitals" or "clouds", but for GCSE shells are fine.
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