Atomic structure, isotopes and the development of the atomic model
WJEC Physics asks both for current atomic structure and for how the model evolved over time.
Modern atomic structure
An atom has a tiny dense nucleus made of protons and neutrons, with electrons orbiting in shells.
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | 1/1836 (~0) | -1 | Shells |
- Atomic (proton) number, Z — number of protons. Defines the element.
- Mass number, A — protons + neutrons.
- Electrons in a neutral atom = Z.
- The nucleus contains over 99.95% of the mass but only ~1/100,000 of the diameter.
Isotopes
Isotopes are atoms of the same element with the same proton number but different numbers of neutrons. They have identical chemistry but different masses, and some are radioactive.
Example: carbon-12 (6p, 6n) and carbon-14 (6p, 8n). Carbon-14 is unstable and used in radiocarbon dating.
How the model developed
- Dalton (early 1800s) — atoms are tiny indivisible spheres.
- Thomson (1897) — discovered the electron; proposed the "plum pudding" model: positive sphere with embedded electrons.
- Rutherford (1909, gold-foil/alpha-scattering) — most alpha particles passed through, a few deflected sharply. Conclusion: atom is mostly empty space with a small dense positive nucleus. This replaced the plum pudding model.
- Bohr (1913) — electrons orbit in fixed shells/energy levels; explained line spectra.
- Chadwick (1932) — discovered the neutron, completing the modern picture.
WJEC exam tip
When asked to describe how Rutherford's experiment changed the model, name the observation (most alphas passed straight through, a few deflected at large angles) AND the conclusion (mostly empty space, small dense positive nucleus). Both halves are needed for full credit.
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