Atomic structure and isotopes
The atom
An atom has a tiny dense nucleus of protons and neutrons, surrounded by electrons in shells.
| Particle | Charge | Relative mass | Location |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | ≈ 1/1836 (≈ 0) | Shells |
Atoms are neutral overall — number of protons = number of electrons.
Atomic and mass numbers
- Atomic (proton) number Z: number of protons. Defines the element.
- Mass number A: total of protons and neutrons.
Number of neutrons = A − Z.
Isotopes
Isotopes of an element have the same number of protons but different numbers of neutrons — same Z, different A. Examples:
- Carbon-12 (6p, 6n) and carbon-14 (6p, 8n).
- Chlorine-35 and chlorine-37.
Isotopes have identical chemical properties (same electron arrangement) but slightly different physical properties (different mass).
Development of the atomic model
| Date | Model | Key contributor | Idea |
|---|---|---|---|
| 1803 | Solid sphere | Dalton | Atoms are indivisible balls |
| 1897 | Plum-pudding | J.J. Thomson | Electrons embedded in positive sphere |
| 1909 | Nuclear | Rutherford | Gold-foil experiment showed positive nucleus |
| 1913 | Shells | Bohr | Electrons orbit in fixed energy levels |
| Modern | Quantum / cloud | Schrödinger and others | Electrons in probability orbitals; protons + neutrons in nucleus (Chadwick 1932) |
Rutherford's gold-foil experiment
Most α particles passed straight through → atom mostly empty space. A few were deflected → small positive nucleus. A tiny number bounced back → nucleus is dense and positively charged. This demolished the plum-pudding model.
Atomic radius scale
A nucleus is about 10⁻¹⁵ m wide; a whole atom about 10⁻¹⁰ m. The nucleus is roughly 1/100 000 the size of the atom — like a pea in the centre of a football pitch.
Edexcel exam tip
For "describe how Rutherford's experiment changed the model" (3 marks): observation → conclusion → impact on previous model. Skip any one of these and you lose a mark.
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