Course: Combined Science + Separate Physics | Tier: Foundation | Linked: Atoms and isotopes notes
✏️ You need: your exercise book and a calculator. Write your answers before you open them.
📌 KEY DEFINITIONS – learn them word for word
Plum pudding model: a ball of positive charge with negative electrons stuck in it.
Nuclear model: a tiny, positive nucleus in the centre, with electrons around it. Most of the atom is empty space.
Scientific models change when new evidence is found that the old model cannot explain.
🎬 Watch: Free Science Lessons videos
Read the story below first. Then watch the Free Science Lessons videos on the atomic model and the alpha scattering experiment, and add anything new to your timeline.
Opens YouTube in a new tab. Choose the video from the Freesciencelessons channel.
📚 Story: The Atom Detectives
For over 2000 years, people wondered: what is everything made of? This is the story of the detectives who found out – and how every new clue changed the answer.
Chapter 1 – The big idea (about 400 BC) 💭
In ancient Greece, a thinker called Democritus asked a simple question. What if you kept cutting something in half, again and again? He thought you would reach a piece so tiny it could not be cut any more. He called it atomos, which means “uncuttable”. He had no experiments – just an idea. (You don’t need Democritus for the exam, but it’s where the word “atom” comes from!)
Chapter 2 – Tiny solid balls (early 1800s) ⚪
Over 2000 years later, an English teacher called John Dalton did experiments with gases and chemicals. His results made sense if everything was made of tiny atoms. He pictured them as solid balls that could not be split. Each element had its own kind of ball.
👉 Model 1: the solid sphere.
Chapter 3 – A surprise inside (1897) 🍮
J.J. Thomson was experimenting with electricity in glass tubes. He found tiny particles with a negative charge. They were much, much smaller than an atom. He had discovered the electron!
This was a shock. It meant atoms could be split into smaller pieces. Thomson knew atoms have no overall charge, so there must also be some positive charge to cancel out the electrons. He imagined the atom as a ball of positive charge with negative electrons stuck in it – like fruit in a Christmas pudding.
👉 Model 2: the plum pudding model.
Chapter 4 – The shocking experiment (1909–1911) ☀️
Ernest Rutherford gave two of his team, Hans Geiger and Ernest Marsden, a job. They fired tiny, positive alpha particles at a sheet of gold foil only a few atoms thick.
If the plum pudding model was right, the positive charge was spread out thinly. So the alpha particles should all go straight through.
Most of them did go straight through. Some were bent to the side. And a very few bounced straight back! Rutherford was amazed. He said it was like firing a huge shell at a sheet of tissue paper – and having it bounce back and hit you.
The clue was clear. The positive charge and nearly all the mass must be squashed into a tiny centre – the nucleus. The rest of the atom is mostly empty space. The plum pudding model could not explain this new evidence, so it had to go.
👉 Model 3: the nuclear model.
Chapter 5 – Electrons in lanes (1913) 🎯
A Danish scientist, Niels Bohr, improved the model. He said electrons move around the nucleus at fixed distances – like runners who must stay in their own lanes on a running track. These lanes are called shells (or energy levels). His calculations matched what other scientists saw in their experiments, so his idea was accepted.
👉 Model 4: electrons in shells.
Chapter 6 – The missing piece (1932) ⚛️
Scientists had found that the nucleus contains positive protons. But there was a puzzle: the nucleus was too heavy to be made of protons alone. About 20 years after the nucleus was discovered, James Chadwick solved it. He found a new particle with no charge and about the same mass as a proton – the neutron.
👉 The atom today: protons and neutrons in a tiny nucleus, with electrons in shells around it.
The end… for now 🔍
Every time someone found new evidence that the old model could not explain, scientists changed the model. That is how science works. And the story isn’t finished – scientists are still discovering new things about the atom today!
✏️ Story task: draw a timeline across two pages of your book with 6 boxes – one for each chapter. In each box, write the year, the scientist, and draw their model.
Quick check – answer in your book:
1. What does atomos mean?
2. What did Thomson discover?
3. Why did the plum pudding need some positive charge?
4. What surprised the scientists in the gold foil experiment?
5. What did Chadwick discover, and when?
Show answers
1. Uncuttable
2. The electron
3. Atoms have no overall charge, so positive charge is needed to cancel the negative electrons
4. A very few alpha particles bounced straight back
5. The neutron, in 1932
📖 Notes: the story of the atom
| When | Who | What they found | The model |
|---|---|---|---|
| Early 1800s | John Dalton | Everything is made of tiny atoms | ⚪ Atoms are tiny solid spheres that cannot be split |
| 1897 | J.J. Thomson | Discovered the electron | 🍮 Plum pudding: positive ball with electrons stuck in it |
| 1909–1911 | Rutherford (with Geiger and Marsden) | Alpha scattering experiment | ☀️ Nuclear model: tiny positive nucleus, mostly empty space |
| 1913 | Niels Bohr | Electrons orbit at fixed distances | 🎯 Electrons in shells (energy levels) |
| 1932 | James Chadwick | Discovered the neutron | ⚛️ Nucleus contains protons and neutrons |
The alpha scattering experiment
Scientists fired tiny positive alpha particles at a very thin sheet of gold foil. This is what happened:
| What they saw | What it showed |
|---|---|
| ➡️ Most went straight through | Most of the atom is empty space |
| ↗️ Some were deflected (bent) | The centre of the atom is positive (it pushes the positive alpha particles away) |
| ↩️ A very few bounced straight back | The mass is packed into a tiny nucleus |
The plum pudding model could not explain the bounce-backs, so it was replaced by the nuclear model.
The atom today
| Particle | Where it is | Charge | Relative mass |
|---|---|---|---|
| Proton | nucleus | +1 | 1 |
| Neutron | nucleus | 0 | 1 |
| Electron | shells around the nucleus | −1 | very small |
An atom has no overall charge because it has the same number of protons and electrons.
An atom is about 0.1 nanometres across (radius about 1 × 10−10 m). The nucleus is about 10 000 times smaller than the atom.
🧮 How to count the particles – 3 steps
Every element has two numbers:
• Atomic number (the small number) = number of protons
• Mass number (the big number) = protons + neutrons
Step 1 – Protons = atomic number
Step 2 – Electrons = same as protons (in an atom)
Step 3 – Neutrons = mass number − atomic number
✅ Worked example
Sodium has atomic number 11 and mass number 23.
Protons = 11
Electrons = 11
Neutrons = 23 − 11 = 12
🔍 See diagrams of the atomic models · See the alpha scattering experiment
✏️ Questions
Level 1: The story
Q1 Put these models in the order they were suggested: nuclear model · solid sphere · plum pudding
Show answer
1. solid sphere → 2. plum pudding → 3. nuclear model
Q2 Match each scientist to their discovery:
Thomson · Chadwick · Bohr
electrons in shells · the neutron · the electron
Show answer
Thomson → the electron
Chadwick → the neutron
Bohr → electrons in shells
Q3 🍮 Describe the plum pudding model.
Show answer
A ball of positive charge with negative electrons stuck in it.
Level 2: The alpha scattering experiment
Q4 Most alpha particles went straight through the gold foil. What does this tell us?
Show answer
Most of the atom is empty space.
Q5 A very few alpha particles bounced straight back. What does this tell us?
Show answer
The mass of the atom is packed into a tiny nucleus in the centre.
Q6 Why was the plum pudding model replaced?
Show answer
The new evidence from the alpha scattering experiment (particles bouncing back) could not be explained by the plum pudding model.
Level 3: Count the particles
Q7 Lithium has atomic number 3 and mass number 7. How many protons, electrons and neutrons does it have?
Show answer
Protons = 3
Electrons = 3
Neutrons = 7 − 3 = 4
Q8 Carbon has atomic number 6 and mass number 12. How many protons, electrons and neutrons does it have?
Show answer
Protons = 6
Electrons = 6
Neutrons = 12 − 6 = 6
Q9 Gold has atomic number 79 and mass number 197. How many neutrons does a gold atom have?
Show answer
Neutrons = 197 − 79 = 118
Q10 Why does an atom have no overall charge?
Show answer
It has the same number of protons (+) and electrons (−), so the charges cancel out.
Level 4: Challenge – how small?
Q11 🏟️ The nucleus is about 10 000 times smaller than the atom. If an atom were blown up to the size of a football stadium 100 m across, how big would the nucleus be?
Hint: divide 100 m by 10 000.
Show answer
100 ÷ 10 000 = 0.01 m = 1 cm – about the size of a pea in the middle of the stadium! That shows how much of the atom is empty space.
Q12 An atom of sodium (11 protons) loses 1 electron. How many electrons does it have now, and what is its charge?
Show answer
Electrons = 11 − 1 = 10
11 positive protons and 10 negative electrons, so the charge is +1. It is now a positive ion.
⚠️ Where students lose marks
- Saying the plum pudding model has a nucleus. It does not – the positive charge is spread through the whole ball.
- Mixing up the observations: most went through = empty space; bounced back = tiny, heavy nucleus.
- Getting the neutron sum wrong: neutrons = mass number − atomic number (big number − small number).
- Saying scientists changed the model because they “changed their minds”. Say: new evidence was found.
Next: Atoms and isotopes notes · Radioactive decay · Atomic Structure topic page
