The shape of the lesson
By the end of the lesson, learners can
- Watch the scatter: Fire alpha particles at the foil and see how most pass through while a rare few rebound.
- From results to model: Each observation points to a feature of the atom.
- Straight through: atom is mostly empty space.
- Large deflection: concentrated positive charge.
- Bounce back: tiny, dense nucleus with most of the mass.
One picture every idea returns to
When alpha particles are fired at thin gold foil, most pass straight through, a few are deflected, and very few bounce back. This shows the atom is mostly empty space with a tiny, dense, positively charged nucleus that holds most of the mass.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Firing at the unseen.. Fire a stream of alpha particles at a gold film so thin it is almost transparent. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Watch the scatter | Fire alpha particles at the foil and see how most pass through while a rare few rebound. Project tab 4 of the lab (Alpha scattering) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: From results to model | Each observation points to a feature of the atom. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 12 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in From results to model, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The rare alpha particles that bounce almost straight back are the key evidence. | They show the positive charge and the mass are concentrated in a tiny nucleus, not spread thinly across the whole atom. |
The alpha-scattering experiment: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the alpha-scattering experiment. State what is fired, what it is fired at, and what is measured.
Most of the alpha particles passed straight through the foil with little or no deflection. State what this shows about the atom.
A small number of alpha particles were deflected through large angles. Explain what this tells us, and why the alpha particles were pushed away.
A very few alpha particles bounced almost straight back towards the source. State the two things this tells us about the nucleus.
An alpha particle is fired straight towards the centre of a nucleus. Describe its path and explain what happens.
Before this experiment, some scientists pictured the atom as positive charge spread evenly through a soft ball. Explain how the bounce-back result shows that this picture must be wrong.
Answer key and marking notes
- A beam of positive alpha particles is fired. ✓
- It is fired at a very thin gold foil. ✓
- The directions in which the alpha particles travel after hitting the foil are measured (the angles they are deflected). ✓
The atom is mostly empty space. ✓
- There is a concentrated positive charge in the atom (the nucleus). ✓
- The alpha particle is positive, so it is repelled by the positive nucleus. ✓
- The nucleus is very small. ✓
- The nucleus is very dense and contains almost all the mass of the atom. ✓
- It slows down as it approaches, stops, and is pushed back the way it came (bounces back). ✓
- The strong repulsion between the positive nucleus and the positive alpha particle turns it around. ✓
- Spread-out charge would only bend the fast alpha particles slightly, never turn them back. ✓
- A bounce-back needs the charge and mass concentrated in a tiny, dense nucleus. ✓
5 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
In the alpha-scattering experiment, most alpha particles passed straight through the thin gold foil with little or no deflection. What does this suggest about the atom?
- AThe atom is mostly empty space.
- BThe atom is a solid ball of matter.
- CAlpha particles have no mass.
- DGold atoms have no nucleus.
In the same experiment, a very small number of alpha particles were deflected through large angles, some bouncing almost straight back. What does this show?
- AThe atom has a tiny, dense, positively charged nucleus.
- BThe atom is uniformly positive throughout.
- CThe nucleus is negatively charged.
- DElectrons are very heavy.
Why are some alpha particles deflected by the gold nucleus?
- AThe positive nucleus repels the positive alpha particle.
- BThe negative nucleus attracts the alpha particle.
- CGravity between them pulls the alpha particle in.
- DThe alpha particles collide with electrons.
Why was the alpha-scattering experiment carried out in a vacuum?
- Aso that air molecules would not absorb or scatter the alpha particles
- Bto stop the foil from melting
- Cto remove the force of gravity
- Dto keep the detector cool
The alpha-scattering results replaced an earlier model in which positive charge was spread evenly through the atom. What model replaced it?
- Athe nuclear model, with charge and mass concentrated in a tiny central nucleus
- Ba model with electrons at the centre
- Ca model with no charged particles
- Da model where the atom is completely solid
Quiz key
Most alpha particles passing straight through shows that the atom is mostly empty space. A solid ball of matter would have deflected or stopped many more of them.
The rare large-angle deflections show a tiny, dense, positively charged nucleus that strongly repels the positive alpha particles when they pass close to it. Because such deflections are rare, the nucleus must be very small.
The nucleus and the alpha particle are both positively charged, so they repel by the electrostatic force, deflecting the alpha particle. The light electrons are far too small in mass to cause large deflections.
Alpha particles are easily absorbed and scattered by air, so a vacuum ensures they reach the foil and detector without being affected first. This makes the deflections caused by the foil reliable to measure.
The results led to the nuclear model, in which the positive charge and nearly all the mass are concentrated in a tiny central nucleus, with electrons orbiting at a distance. The earlier even-spread (plum pudding) model could not explain the large-angle deflections.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.