Welcome! Nobody has ever seen inside an atom with their eyes – so how do we know what’s in there? In this lesson you’ll be a science detective, using real experimental evidence to work out how our model of the atom changed. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper and a pen.
- Write the title and date: The model of the atom – Science Detectives.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. describe the plum pudding model and the nuclear model
2. explain what the alpha scattering experiment showed
3. explain why scientific models change.
Step 1: Starter (5 minutes)
Draw what you think an atom looks like. Label as many parts as you can. Keep it – you’ll come back to it at the end!
Show answer
Check your drawing against the box below. It took scientists over 100 years to work this out!
📝 Copy into your book
Today’s model of the atom: a tiny, positively charged nucleus containing protons and neutrons, with electrons in energy levels around it. Most of the atom is empty space.
Step 2: Read and write (10 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- What did the plum pudding model say an atom was like?
- What were alpha particles fired at in the scattering experiment?
- What did Bohr add to the model?
📖 Read: How our model of the atom changed
Scientists once thought atoms were tiny spheres that couldn’t be divided. Then the electron was discovered, which showed atoms contain even smaller particles. This led to the plum pudding model: the atom was a ball of positive charge with negative electrons embedded in it, like fruit in a pudding.
In the alpha scattering experiment, scientists fired alpha particles at a very thin sheet of gold foil. Most went straight through, but a few were deflected and some even bounced back. This showed that the mass of an atom is concentrated in a tiny, positively charged nucleus, and most of the atom is empty space – the nuclear model. Later, Niels Bohr suggested that electrons orbit the nucleus at specific distances, called energy levels. Later still, protons were found in the nucleus, and James Chadwick showed that neutrons are there too.
Show answers
(a) A ball of positive charge with negative electrons embedded in it.
(b) A very thin sheet of gold foil.
(c) Electrons orbit the nucleus at specific distances – in energy levels (shells).
📝 Copy and complete
The plum pudding model said the atom was a ball of __________ charge with __________ electrons embedded in it. It was replaced after the __________ __________ experiment.
Check your gaps
positive · negative · alpha scattering
Step 3: The timeline (8 minutes)
Your task: draw a timeline across your page with five boxes. Open each card and fill in one box with a name, a quick sketch and one key idea.
1. Tiny spheres
Early scientists thought atoms were tiny solid spheres that couldn’t be divided into anything smaller.
2. The plum pudding model
The discovery of the electron showed atoms could be divided. The new model: a ball of positive charge with negative electrons scattered through it, like fruit in a pudding.
3. The nuclear model
The alpha scattering experiment showed the mass of the atom is concentrated in a tiny, positively charged nucleus at the centre, with the electrons around it. Most of the atom is empty space.
4. Bohr’s model
Niels Bohr suggested that electrons orbit the nucleus at specific distances (energy levels). His calculations agreed with experimental results.
5. Protons and neutrons
Later experiments showed the positive charge of the nucleus is made of smaller particles – protons. About 20 years after the nucleus was accepted, James Chadwick proved that neutrons exist in the nucleus too.
Underneath your timeline, copy this summary:
📝 Copy into your book
tiny spheres → plum pudding (electron discovered) → nuclear model (alpha scattering) → Bohr’s energy levels → protons → neutrons (Chadwick)
✅ Move on when your timeline and summary are done.
Step 4: Main task – Science Detectives (12 minutes)
The case: scientists fired positively charged alpha particles at a very thin sheet of gold foil. If the plum pudding model was right, the alpha particles should all pass straight through with only tiny deflections. Here’s what they actually saw:
| Clue | Observation |
|---|---|
| Clue 1 | Most alpha particles went straight through the foil. |
| Clue 2 | Some alpha particles were deflected (changed direction). |
| Clue 3 | A very small number bounced straight back. |
Your task: for each clue, write “This tells me that…” and explain what it proves about the atom. Then write a verdict: was the plum pudding model right or wrong, and why?
Stuck? Alpha particles are positive. What happens when two positive charges get close? What would it take to make something bounce straight back?
Show the detective’s report
Clue 1: most of the atom is empty space.
Clue 2: the centre of the atom is positively charged – it repels the positive alpha particles that pass close to it.
Clue 3: the mass (and charge) is concentrated in a tiny nucleus – only something small and very dense could bounce an alpha particle straight back.
Verdict: the plum pudding model was wrong. It predicted that all the alpha particles would go straight through, but some bounced back. The evidence couldn’t be explained by the old model, so it was replaced by the nuclear model.
📝 Copy into your book
Alpha scattering showed:
Most went straight through → most of the atom is empty space.
Some were deflected → the nucleus is positively charged.
A few bounced back → the mass is concentrated in a tiny nucleus.
Step 5: Exam practice (15 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (2 marks) Describe the plum pudding model of the atom.
Show model answer
A ball of positive charge (1) with negative electrons embedded in it (1).
Q2 (4 marks) Compare the plum pudding model with the nuclear model of the atom.
Hint: use whereas in every point. Think about where the mass is, where the positive charge is, where the electrons are, and empty space.
Show model answer
In the plum pudding model the mass is spread through the whole atom, whereas in the nuclear model it is concentrated in the nucleus (1). In the plum pudding model the positive charge is spread out, whereas in the nuclear model it is in the nucleus (1). In the plum pudding model the electrons are embedded in the positive charge, whereas in the nuclear model they are around the nucleus (in energy levels) (1). The nuclear model is mostly empty space, whereas the plum pudding model has no empty space (1).
Q3 (6 marks) Describe how and why the model of the atom has changed over time.
Hint: use your timeline summary! For each step, say what the model was and what new evidence changed it.
Show model answer
- Atoms were first thought to be tiny spheres that could not be divided.
- The discovery of the electron showed atoms contain smaller particles, leading to the plum pudding model: a ball of positive charge with electrons embedded in it.
- In the alpha scattering experiment, most alpha particles passed through gold foil but a few bounced back. This couldn’t be explained by the plum pudding model.
- So it was replaced by the nuclear model: the mass and positive charge are concentrated in a tiny nucleus, and most of the atom is empty space.
- Bohr suggested electrons orbit in energy levels at specific distances; his calculations agreed with experiments.
- Later experiments found protons in the nucleus, and Chadwick showed neutrons exist. Models change whenever new evidence can’t be explained by the old one.
📝 Copy into your book
Scientific models change when new evidence can’t be explained by the old model.
✅ Add up your marks out of 12 and write the score in your book.
Step 6: Exit ticket (10 minutes)
Go back to your drawing from Step 1. Draw a new atom next to it using everything you’ve learned, and label: nucleus, protons, neutrons, electrons, energy levels, empty space. Then finish:
- My drawing changed because…
- The most important experiment was… because…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Atoms, isotopes and the model of the atom questions.
🖨️ Want this lesson on paper? Download the whole lesson as a printable worksheet – answers are on the last page.
