The Nuclear Model of the Atom · revision map · IGCSE 0625, Topic 5.1
TOPIC 5.1 · THE NUCLEAR MODEL OF THE ATOM
CAMBRIDGE IGCSE PHYSICS 0625 · ONE-PAGE REVISION MAP
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THE ATOM5.1
The Nuclear Model
An atom has a tiny, dense, positive nucleus at its centre, with electrons moving around it in shells.
- The nucleus holds nearly all the mass.
- Electrons are negative; the atom is neutral overall.
- Most of the atom is empty space.
Protons, Neutrons & Electrons
The nucleus is made of protons and neutrons (together: nucleons). Electrons orbit outside.
| Particle | Rel. charge | Rel. mass |
|---|---|---|
| Proton | +1 | 1 |
| Neutron | 0 | 1 |
| Electron | −1 | 1⁄1840 |
- Protons and neutrons have almost equal mass.
- The electron mass is negligible by comparison.
- An atom is neutral: protons = electrons.
Proton & Nucleon Number
Proton number Z = number of protons (it decides the element).
Nucleon number A = protons + neutrons.
- Neutrons = A − Z.
- A neutral atom has Z electrons.
- e.g. carbon: A = 12, Z = 6.
Isotopes
Isotopes are atoms of the same element with different numbers of neutrons: same Z, different A.
- They behave the same chemically.
- Example: carbon-12 and carbon-14.
The Alpha-Particle Scattering EvidenceEXTENDED
Firing alpha particles at thin gold foil revealed the nuclear model:
- Most passed straight through → the atom is mostly empty space.
- A few were deflected through large angles → the nucleus is positively charged.
- Very few bounced back → the nucleus is tiny and very dense, holding most of the mass.