P4.1 Atoms and Isotopes
Structure of the atom
Every atom has a nucleus (tiny, dense, positively charged) surrounded by electrons (negatively charged, very low mass, in shells/energy levels).
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (~0) | −1 | Shells around nucleus |
The nucleus occupies a tiny fraction of the atom's volume — approximately 1/10,000 of the atomic radius — yet contains almost all the mass.
Atomic number, mass number and isotopes
Atomic number (Z): number of protons in the nucleus. Defines which element it is.
Mass number A: total number of protons + neutrons (nucleons).
Number of neutrons = A − Z
Standard notation:
$$^{A}_{Z}X$$ where X is the chemical symbol.
Isotopes
Isotopes are atoms of the same element (same Z) with different numbers of neutrons (different A).
- They have identical chemical properties (same number of electrons → same bonding behaviour).
- They have different physical properties (different mass; some are radioactive).
Example — Carbon isotopes:
| Isotope | Z | A | Neutrons | Stable? |
|---|---|---|---|---|
| ¹²C | 6 | 12 | 6 | Yes |
| ¹³C | 6 | 13 | 7 | Yes |
| ¹⁴C | 6 | 14 | 8 | No — radioactive; used in carbon dating |
Example — Uranium isotopes:
| Isotope | Z | A | Neutrons |
|---|---|---|---|
| ²³⁵U | 92 | 235 | 143 |
| ²³⁸U | 92 | 238 | 146 |
Development of the atomic model
The model of the atom has changed as new experimental evidence emerged:
- Solid sphere model (Dalton, ~1800): Atoms as indivisible solid balls.
- Plum pudding model (Thomson, 1897): Electrons embedded in a positive "pudding" — after discovery of the electron.
- Nuclear model (Rutherford, 1911): Geiger–Marsden gold foil experiment showed that most alpha particles pass straight through, but a few are deflected at large angles (including back-scatter). Conclusion: mass and positive charge concentrated in a tiny nucleus; most of the atom is empty space.
- Bohr model (1913): Electrons orbit in fixed energy levels (shells). Explained line spectra. Electrons can jump between levels by absorbing/emitting photons.
- Modern quantum model: Electrons described by probability distributions (orbitals), not fixed orbits. Beyond GCSE detail.
Rutherford's gold foil experiment — key observations
- Most alpha particles passed straight through → atom is mostly empty space.
- Small fraction deflected slightly → nucleus is positively charged.
- Very small fraction deflected through large angles / bounced back → nucleus is tiny and very dense.
Common exam errors
- Saying mass number = number of neutrons. It's protons + neutrons.
- Confusing isotopes with different elements. Same element = same Z.
- Saying the Rutherford experiment "disproved" the plum pudding model without explaining what the evidence showed.
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