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The early model pictured the atom as an indivisible solid sphere, while the plum pudding model pictured a positive sphere with embedded electrons. Free notes.
Early Atomic Models is part of Atoms and isotopes in GCSE AQA Higher Physics.
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The early model pictured the atom as an indivisible solid sphere, while the plum pudding model pictured a positive sphere with embedded electrons. Free notes.
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Use this lesson to stay inside Atoms and isotopes while connecting the detail back to the wider Atomic structure topic.
Read the core explanations from Early Atomic Models before testing yourself from memory.
For centuries, scientists wondered: what are things actually made of? If you kept cutting a piece of gold into smaller and smaller pieces, would you eventually reach something that couldn't be cut any further?
The ancient Greeks had a name for this ultimate particle - the 'atom', meaning 'uncuttable'. But it took until the 1800s for scientists to find real evidence that atoms existed.
Before 1897, scientists had a beautifully simple picture of atoms. They thought atoms were tiny, solid spheres - like incredibly small ball bearings. These spheres were completely indivisible and indestructible.
This model explained a lot. Different elements had different types of atoms - hydrogen atoms were the lightest spheres, oxygen atoms were heavier, and so on. Chemical reactions happened when these spheres rearranged themselves into new combinations.
In 1897, J.J. Thomson was experimenting with cathode rays - mysterious beams that appeared in glass tubes when electricity passed through them. He discovered something shocking: these rays were made of particles much smaller than atoms.
This discovery shattered the 'indivisible sphere' model. If electrons could be pulled out of atoms, then atoms weren't solid spheres at all - they had internal structure. But this created a new puzzle: atoms are normally neutral, so if they contain negative electrons, where's the positive charge?
Thomson needed a new model that could explain both electrons and the fact that atoms are normally neutral. His solution was ingenious: what if atoms were spheres of positive charge with negative electrons dotted throughout them?
Picture a Christmas pudding with raisins scattered through it. In Thomson's model, the pudding represents positive charge spread throughout the atom, and the raisins represent the negative electrons. The positive and negative charges balance out, making the atom neutral overall.
This model explained how atoms could contain electrons while still being neutral. It also suggested that electrons could be removed (creating positive ions) or added (creating negative ions) during chemical reactions.
| Feature | Early Model | Plum Pudding Model |
|---|---|---|
| Structure | Solid sphere | Positive sphere with embedded electrons |
| Divisibility | Indivisible | Contains smaller particles |
| Charge | Neutral (no internal charges) | Neutral (balanced positive and negative) |
| Electrons | Not known to exist | Embedded throughout the atom |
The plum pudding model was a huge step forward. It was the first atomic model to include subatomic particles and explain how atoms could have internal structure while remaining neutral. However, it wouldn't last long - new experiments were already being planned that would reveal even more surprises about atomic structure.
Exam questions on early atomic models often test your ability to describe and compare different models. The key is being precise about what each model proposed and when it was developed.
Worked example: Worked Method: Early Atomic Models
Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.
Answer: Final answer should include unit and appropriate significant figures.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
J.J. Thomson discovered the electron in 1897 while experimenting with cathode rays in glass tubes. He found the rays were made of particles much smaller than atoms: negatively charged electrons about 1800 times lighter than a hydrogen atom.
The electron was discovered in 1897 by J.J. Thomson, who was experimenting with cathode rays. He found that the rays were made of particles much smaller than atoms.
The word atom comes from the ancient Greeks and means 'uncuttable'. It was their name for the ultimate particle that could not be cut any further, though real evidence that atoms existed only came in the 1800s.
Cathode rays are beams that appear in glass tubes when electricity passes through them. In 1897 J.J. Thomson found they were made of particles much smaller than atoms, which were electrons.
The discovery of the electron shattered the indivisible sphere model, because if electrons could be pulled out of atoms, atoms had internal structure. Thomson then proposed the plum pudding model to explain how atoms contain negative electrons yet are normally neutral.
The early model pictured the atom as an indivisible solid sphere with no internal charges, while the plum pudding model pictured a positive sphere with embedded electrons. The plum pudding model was the first atomic model to include subatomic particles.
An electron is about 1800 times lighter than a hydrogen atom. It is a negatively charged subatomic particle, discovered by Thomson, and found in all elements.
Thomson proposed the plum pudding model to explain both electrons and the fact that atoms are normally neutral. If atoms contained negative electrons, there had to be positive charge too, so he pictured a sphere of positive charge with electrons dotted throughout.
The plum pudding model explained how atoms could contain electrons while still being neutral. It also suggested electrons could be removed to create positive ions, or added to create negative ions, and it was the first model to include subatomic particles.
The solid sphere model had to change because the discovery of the electron in 1897 showed atoms had internal structure. If electrons could be pulled out of atoms, atoms could not be solid, indivisible spheres.
In the early atomic model, chemical reactions happened when the tiny, solid spheres rearranged themselves into new combinations. Each element had its own type of sphere with a specific mass, with hydrogen atoms the lightest.
In the plum pudding model, the raisins represent the negative electrons and the pudding represents positive charge spread throughout the atom. The positive and negative charges balance out, making the atom neutral overall.