Models of the atom (AQA GCSE Physics): Revision Notes
Models of the atom
Understanding how scientists' ideas about atoms have changed over time is crucial for GCSE chemistry. Three main models show this development clearly and demonstrate how scientific understanding builds upon previous discoveries.
How atomic models developed
Scientists built their understanding of atoms step by step. Each new model came from fresh evidence and experiments. When new discoveries were made, the old models had to be updated or replaced completely. This shows how science progresses - theories are constantly tested and improved when new evidence emerges.
Plum pudding model
Who created it: J.J. Thomson (after discovering electrons)
What it suggested: Thomson thought atoms were like a Christmas pudding. The atom had a positive background (like the pudding mixture) with tiny negative electrons scattered throughout it (like raisins in a pudding).
Key features:
- Positive and negative charges spread throughout the atom
- No empty space inside atoms
- Electrons mixed evenly in the positive material
This was the first model to include electrons, making it a crucial step forwards in atomic theory, even though it turned out to be incorrect.
Rutherford's model
Who created it: Ernest Rutherford (with help from Geiger and Marsden)
The Gold Foil Experiment
Setup: Rutherford's team fired tiny positive particles (alpha particles) at very thin gold foil. They expected the particles to pass straight through if Thomson's model was correct.
Observations:
- Most particles went straight through (indicating lots of empty space)
- Some particles bounced back at large angles (indicating something dense and positive in the centre)
Conclusion: This unexpected result completely changed our understanding of atomic structure.
What the new model suggested:
- Atoms have a tiny, dense, positive centre called the nucleus
- Electrons move around this nucleus (like planets around the sun)
- Most of the atom is empty space
This was a huge change from the plum pudding model. About 20 years later, scientists also discovered neutrons - neutral particles in the nucleus alongside the positive protons.
The Bohr model
Who created it: Niels Bohr
What it added: Bohr kept Rutherford's nuclear idea but made an important improvement. He showed that electrons don't just orbit anywhere around the nucleus.
Key features:
- Electrons orbit the nucleus in specific energy levels (also called shells or orbits)
- Electrons can only exist in these fixed energy levels
- Electrons can jump between energy levels if they gain or lose the right amount of energy
Why it was accepted: Bohr's mathematical calculations matched what scientists observed in experiments. This gave confidence that his model was correct and showed how scientific theories must be supported by both mathematical predictions and experimental evidence.
Comparing the models
Plum pudding vs Rutherford: The plum pudding model had positive charge spread throughout the atom. Rutherford's model concentrated all positive charge in a tiny central nucleus.
Rutherford vs Bohr: Rutherford thought electrons could orbit at any distance from the nucleus. Bohr showed electrons can only orbit at specific energy levels.
Each model represented the best understanding at the time, but had to be modified when new experimental evidence emerged. This is how scientific knowledge progresses - through continuous testing and refinement of ideas.
Key Points to Remember:
- Thomson's plum pudding model: Electrons scattered in positive "dough" - like raisins in a pudding
- Rutherford's nuclear model: Tiny positive nucleus in centre with electrons orbiting around it - proved by gold foil experiment
- Bohr's model: Electrons orbit in specific energy levels only - can jump between levels
- Each new model: Built on previous ideas but fixed problems when new evidence appeared
- Scientific progress: Models change when new experiments give different results than expected