AQA Physics 6.4.1.3
Development of the model of the atom
One experiment broke a model. Predict what alpha particles should do, see what they did, and follow how each new piece of evidence changed the atom.
Combined Science · Foundation tier
Start here
Fog, or a hidden wall?
You throw tennis balls, one after another, into a thick bank of fog. Somewhere in the fog there may be a brick wall you can’t see.
If you had to guess, what would send a ball straight back to you?
Before the experiment
From solid spheres to plum pudding.
Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided. The discovery of the electron led to the plum pudding model: the atom is a ball of positive charge with negative electrons embedded in it.
Alpha particle scattering · predict, then compare
Fire alpha particles at thin gold foil.
Alpha particles are positively charged. A detector around the foil counts where they go. First predict what the plum pudding model says should happen.
Plum pudding: positive charge spread through the whole atom. What should the alpha particles do?
After the experiment · new evidence, new model
The model kept changing.
New experimental evidence may lead to a scientific model being changed or replaced. Step through what came next.
Evidence
No evidence of anything inside atoms.
Model
Atoms are tiny spheres that cannot be divided.
Think again · what they bounced off
“The few alpha particles that bounced back must have hit electrons.”
Your call
ChooseWhy did a very few alpha particles bounce back from the gold foil?
Sort
Which model?
Place each statement with its model.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- Say what each model looked like, in order.
- Explain
- Observation, then what it shows: most went through, so mostly empty space.
- Compare
- Where the positive charge and mass are in each model.
Key fact
Alpha scattering showed that the mass of an atom is concentrated at the centre, in a nucleus, and that the nucleus is charged. The nuclear model replaced the plum pudding model.
Key note · AQA 6.4.1.3
Development of the model of the atom
- Before the electron was discovered, atoms were thought to be tiny spheres that could not be divided.
- Plum pudding: a ball of positive charge with negative electrons embedded in it.
- Alpha scattering: the mass of an atom is concentrated in a charged nucleus. The nuclear model replaced plum pudding.
- Bohr: electrons orbit the nucleus at specific distances; his calculations agreed with experiments.
- The nucleus’s positive charge is a whole number of protons. Chadwick showed neutrons exist, about 20 years later.
- New experimental evidence may lead to a scientific model being changed or replaced.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure what each scattering result shows?
Model and scattering diagrams are schematic, not to scale; the paths show the pattern, not real counts.