Models of the atom (AQA GCSE Physics Combined Science): Revision Notes
Models of the atom
How atomic models developed
Scientists have changed their ideas about atoms over time. This happened because new experiments gave them fresh evidence about how atoms are built.
Each new discovery led to a better model that explained more about atomic structure.
This shows how science works - models are constantly improved as new evidence becomes available. No scientific model is ever considered "final" because there's always potential for new discoveries.
Plum pudding model
Who created it: J.J. Thomson (after discovering electrons)
What it looked like: Thomson thought the atom was like a Christmas pudding. The atom had:
- A uniform lump of positive charge (the "dough")
- Tiny negative electrons scattered throughout (the "plums")
- Equal amounts of positive and negative charge to make the atom neutral
Key idea: Both positive and negative charges existed in atoms, but they were mixed together evenly.
The "plum pudding" analogy was perfect for the time - it helped people visualise how Thomson thought positive and negative charges were distributed throughout the atom, just like plums scattered evenly through Christmas pudding dough.
Rutherford's model
Who created it: Ernest Rutherford
The experiment: Rutherford and his team fired alpha particles (positive charges) at a thin gold foil. They expected most particles to pass straight through if Thomson's model was correct.
The Gold Foil Experiment: Step by Step
Setup: Alpha particles (positive) fired at thin gold foil Expected result: All particles pass through (based on plum pudding model)
Actual results:
- Most alpha particles went straight through
- Some were scattered at small angles
- A few bounced straight back (completely unexpected!)
Conclusion: There must be a tiny, dense, positive centre that repelled the alpha particles
What this proved: The atom must have a tiny, dense, positively charged centre (the nucleus) with electrons orbiting around it - like planets around the Sun.
The key breakthrough was realising that only a tiny, dense, positive nucleus could cause some alpha particles to bounce straight back. This completely overturned the plum pudding model.
New discoveries: Later experiments showed that the positive charge in the nucleus could be split into smaller particles called protons. James Chadwick then discovered neutrons (neutral particles) in the nucleus about 20 years later.
The Bohr model
Who created it: Niels Bohr
Key improvement: Bohr realised that electrons couldn't orbit the nucleus in any random path. Instead:
- Electrons orbit in specific energy levels (like fixed lanes on a running track)
- Electrons can jump between energy levels if they gain or lose the right amount of energy
- Each energy level has a set amount of energy
Why it was accepted: Bohr's calculations matched what scientists observed in experiments, which proved his model was more accurate.
Think of energy levels like the floors in a building - an electron can be on the 1st floor, 2nd floor, or 3rd floor, but never halfway between floors. It needs exactly the right amount of energy to "jump" from one floor to another.
How the models compare
- Plum pudding: Positive and negative charges mixed evenly throughout the atom
- Rutherford: Small positive nucleus with electrons orbiting around it
- Bohr: Same as Rutherford, but electrons orbit in fixed energy levels rather than random paths
Key Points to Remember:
- Atomic models changed as new experimental evidence was discovered
- Thomson's plum pudding model had electrons embedded in positive "dough"
- Rutherford's scattering experiment proved atoms have a small, dense nucleus
- Bohr showed that electrons orbit in specific energy levels, not random paths
- Each new model built on previous discoveries and explained more about atomic structure