AQA Chemistry 5.1.1.3 · Combined and Triple
Development of the model of the atom
Each model of the atom was replaced when new experimental evidence could not be explained by it. Follow the chain from a solid sphere to protons and neutrons.
Combined Science · Higher tier
Start here
Changing your mind.
You are sure a film came out in 2015. A friend shows you the poster: 2017.
If you had to guess, should an idea stay the same, or change when new evidence disagrees with it?
The chain of models
Model, evidence, new model.
Step through. Before each new piece of evidence arrives, predict which part of the old model it breaks.
Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided. Predict: a particle smaller than an atom is found inside atoms. Which part of the old model breaks?
The chemistry page follows the chain of models. The alpha-scattering experiment itself has its own physics lesson: Development of the atomic model.
The evidence card
Three observations, three conclusions.
Alpha particles (positive) were fired at very thin gold foil. Match each observation to what it showed.
Most alpha particles passed straight through the foil.
What did this show?
Some were deflected through large angles.
What did this show?
A very few came straight back.
What did this show?
Think again · which way round?
“Most of the alpha particles bounced back off the foil.”
Your call
ChooseA pupil says most alpha particles bounced back, which proved the nucleus. What is wrong?
Sort
Which model first said this?
Place each statement.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Each model, and the difference between plum pudding and the nuclear model.
- Explain
- Why the model changed: the evidence, and what it could not fit.
- Give
- What Chadwick found, and roughly when.
Key fact
New experimental evidence may lead to a scientific model being changed or replaced. Alpha scattering replaced plum pudding with the nuclear model: mass concentrated at the centre, in a charged nucleus.
Key note · AQA 5.1.1.3 · Combined and Triple
Development of the model of the atom
- New experimental evidence may lead to a scientific model being changed or replaced.
- Before the electron: atoms were tiny spheres that could not be divided. The electron led to the plum pudding model: a ball of positive charge with electrons embedded in it.
- Alpha scattering: most alpha particles passed straight through, some were deflected, a very few came back. Conclusion: the mass of an atom is concentrated at the centre (nucleus) and the nucleus is charged. The nuclear model replaced plum pudding.
- Bohr: electrons orbit the nucleus at specific distances; his theoretical calculations agreed with experimental observations.
- Later: the nuclear charge is made of whole numbers of protons. Chadwick provided the evidence for neutrons, about 20 years after the nucleus was accepted.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Mixing up plum pudding and the nuclear model?
Dates are approximate: the electron 1897, plum pudding about 1904, the nucleus 1911, neutrons 1932.