The Development of the Atomic Model

A scientific model is our best current picture of how something works. When new experimental evidence appears, a model is either adjusted or replaced. The atom is a perfect example: the way scientists pictured it changed several times over about a century as new discoveries were made.

The key idea the exam tests is not just what each model looked like, but why each one changed. Every step forward came from an experiment that the old model could not explain.

This lesson traces that story in order: the earliest solid-sphere idea, the plum pudding model, the nuclear model, Bohr's refinement, and finally the discovery of protons and neutrons. Understanding the sequence lets you explain how each new model corrected the one before it.

Before electrons were discovered, atoms were thought to be tiny solid spheres that could not be divided into anything smaller. The word "atom" itself comes from a Greek word meaning uncuttable.

This model treated the atom as the smallest possible piece of matter, with no internal structure at all. There was no notion of charge inside it, no smaller particles, and nothing to explain.

That picture held until experiments revealed that atoms are not solid or indivisible after all. The discovery of a small, negatively charged particle inside the atom, the electron, showed that atoms must contain smaller parts. Once scientists knew atoms contained negative electrons, they needed a new model that also accounted for positive charge, because whole atoms are neutral. That requirement produced the plum pudding model.

The discovery of the electron led directly to the plum pudding model.

The plum pudding model described the atom as a ball of positive charge with negative electrons embedded throughout it, like currants scattered through a pudding.

This model met the basic requirement of being neutral overall: the negative charge of the electrons was balanced by the surrounding positive charge. In this picture there was no nucleus. The positive charge and the mass were spread evenly through the whole volume of the atom.

The plum pudding model was a reasonable idea for its time, but it made a clear prediction that could be tested. If positive charge is spread thinly across the whole atom, then a fast, positive particle fired at the atom should pass through with only tiny deflections. Testing exactly this prediction is what brought the model down.

To test the plum pudding model, positively charged alpha particles were fired at a very thin sheet of gold foil, and where they ended up was recorded. The plum pudding model predicted they would all pass almost straight through.

The results did not match that prediction:

Observation	What it tells us
Most alpha particles passed straight through	The atom is mostly empty space
Some were deflected through small angles	The centre of the atom is positively charged (it repels positive alpha particles)
A very few bounced almost straight back	The centre is tiny, very dense and holds a large charge and most of the mass

The plum pudding model could not explain particles bouncing back, because a thinly spread positive charge could never repel a fast alpha particle so strongly. This evidence forced a completely new model.

The scattering results led to the nuclear model, which replaced the plum pudding model.

In the nuclear model, the mass and the positive charge of the atom are concentrated in a tiny central nucleus. The atom is mostly empty space, with electrons around the outside.

Comparing the two models directly makes the change clear, and this comparison is a common exam question.

Feature	Plum pudding model	Nuclear model
Positive charge	Spread through the whole atom	Concentrated in a tiny nucleus
Mass	Spread evenly through the atom	Concentrated in the nucleus
Electrons	Embedded within the positive charge	Around the outside of the nucleus
Empty space	None (atom is filled)	Most of the atom

The nuclear model explained the scattering results: a small dense nucleus lets most particles pass through the empty space, while the concentrated positive charge deflects or repels the few that come close.

The nuclear model was refined further as more evidence came in.

Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances, called energy levels, rather than anywhere around it. This mattered because his theoretical calculations agreed with experimental observations, which is strong support for a model.

(Extra context — the experimental work behind the Bohr model is not required by AQA 8463. You only need to know that Bohr proposed fixed-distance electron orbits and that his calculations matched observations.)

Later experiments showed that the positive charge of the nucleus is not a single blob but is made of whole, separate positive particles. These particles were named protons.

Finally, about twenty years after the nucleus was proposed, James Chadwick carried out experiments that provided evidence for the neutron, an uncharged particle in the nucleus. This completed the modern picture of a nucleus made of protons and neutrons.

(Extra context — the detail of Chadwick's experimental work is not required by AQA 8463. You need only know that he provided evidence for the existence of the neutron.)

1. Saying the alpha scattering experiment discovered the electron

The electron was discovered first, which led to the plum pudding model. Alpha scattering came later and led to the nuclear model. Keep the order straight.

2. Getting the scattering observations and conclusions mixed up

Match each observation to its conclusion: most passing through means mostly empty space; a few deflected or bounced back means a tiny, dense, positively charged nucleus. Do not swap them.

3. Forgetting to say why the plum pudding model was rejected

State the reason explicitly: the plum pudding model could not explain why some alpha particles were deflected or bounced back. A model changes because of evidence it cannot account for.

4. Describing Bohr's or Chadwick's experiments in detail

The specification does not require the experimental detail of Bohr's model or Chadwick's work. Just state what each proposed or provided evidence for: Bohr for fixed-distance electron orbits, Chadwick for the neutron.

5. Confusing which model has a nucleus

The plum pudding model has no nucleus; charge and mass are spread out. The nucleus appears only in the nuclear model. This is the central difference between the two.