GCSE-readyHow the model of the atom developed
Our model of the atom has changed as scientists discovered smaller particles inside it. Today's model has a tiny nucleus surrounded by electrons in shells.
From Dalton to Bohr
Thomson (1897): discovered the electron, so atoms must contain smaller particles. His plum pudding model had electrons in a ball of positive charge.
Rutherford (1909 to 1911): fired alpha particles at gold foil. Most went straight through, but a few bounced back. So the atom is mostly empty space with a tiny, dense, positive nucleus.
Bohr (1913): electrons orbit the nucleus in fixed shells (energy levels).
Chadwick (1932): discovered the neutron in the nucleus.
The particles
| Particle | Relative charge | Relative mass | Where |
|---|---|---|---|
| Proton | +1 | 1 | Nucleus |
| Neutron | 0 | 1 | Nucleus |
| Electron | −1 | 1/1836 (very small) | Shells around the nucleus |
Size and mass
The nucleus is very small compared with the whole atom: its radius is about 1/100 000 of the atom's radius.
Almost all the mass of an atom is in the nucleus, because electrons are so light.
An atom has 17 protons and 18 neutrons. How many electrons does it have, and what is its overall charge?
- In an atom, number of electrons = number of protons.
- So there are 17 electrons.
- 17 positive charges and 17 negative charges cancel.
Answer: 17 electrons; no overall charge (neutral).
Dalton: solid spheres. Thomson: electrons, plum pudding. Rutherford: tiny dense positive nucleus. Bohr: electron shells. Protons +1, mass 1; neutrons 0, mass 1; electrons −1, mass 1/1836. The nucleus is tiny but holds almost all the mass.
- Atom
- The smallest particle of an element that can take part in a chemical reaction.
- Electron
- A tiny negative particle (relative charge -1) found in shells around the nucleus.
- Neutron
- A neutral particle (relative mass 1) in the nucleus.
All GCSE and International GCSE Chemistry key terms and definitions →
- Not “1”. That is the mass of a proton or neutron.
- Not “−1”. −1 is its relative charge, not its mass.
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What did the plum pudding model suggest the atom was?
- A solid, indivisible sphere
- A tiny nucleus with electrons in shells
- Mostly empty space
- A ball of positive charge with negative electrons embedded in it
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Answer: A ball of positive charge with negative electrons embedded in it
Thomson proposed the plum pudding model after discovering electrons.
What conclusion did Rutherford draw from alpha particles passing straight through gold foil?
- The nucleus is negatively charged
- The electrons are in the nucleus
- Atoms are solid spheres
- Most of the atom is empty space
Show the answer
Answer: Most of the atom is empty space
Most particles meeting nothing means most of the atom is empty.
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- Pearson Edexcel GCSE Chemistry (1CH0) specification, topic 1.1, 1.2, 1.3, 1.4, 1.5, 1.6
- Nobel Prize: J.J. Thomson facts
- American Physical Society: Rutherford and the discovery of the atomic nucleus
- Nobel Prize: James Chadwick facts
Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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