Year 8 Science · Unit 2 · Lesson 10
Foundation Worksheet
Learning Goals
Fill the gap
Choose the correct word from the word bank. Two words will not be used.
Dalton pictured the atom as a solid with nothing inside it. Thomson discovered the and described the atom as a ball of charge with electrons dotted through it. Rutherford showed that the atom has a tiny, dense and is mostly space. Bohr placed the electrons in fixed around the centre.
1. Thomson’s model is called the plum-pudding model. Explain what the “pudding” and the “plums” stand for.
Put the discoveries in order
Number the events from 1 (earliest) to 6 (latest). Think about which idea each event needed first.
| Event (1–6) | What happened |
|---|---|
| Rutherford proposes that the atom has a tiny, dense, positive nucleus with electrons outside it. | |
| Thomson proposes the plum-pudding model. | |
| Bohr proposes that electrons can only be in fixed shells around the nucleus. | |
| Dalton proposes that atoms are solid spheres that cannot be split. | |
| Alpha particles are fired at thin gold foil, and a few bounce back. | |
| Thomson shows that cathode rays are streams of tiny negative particles. |
Scenario: a gold-foil simulation
A class runs a computer simulation of the gold-foil experiment. It fires 16 000 alpha particles at a thin sheet of gold and counts what happens to each one.
| What the alpha particle did | Number of particles |
|---|---|
| Passed straight through | 15 200 |
| Changed direction slightly | 798 |
| Bounced back towards the source | 2 |
Source: sample results from a classroom simulation.
2. What percentage of the alpha particles passed straight through?
Answer %
3. Only 2 particles bounced back. Explain why even these 2 were enough to show that the plum-pudding model was wrong, and what they tell you about the centre of an atom.
4. Choose two times the atomic model changed. For each one, describe the old model, the new evidence, and the new model. Finish by explaining why the older model was still useful in its time.
Fill the gap
sphere; electron; positive; nucleus; empty; shells. Not used: negative, neutron.
1. The “pudding” is a ball of positive charge that fills the whole atom; the “plums” are the negative electrons dotted through it. [2 marks]
Put the discoveries in order
Reading down the table as printed: 5, 3, 6, 1, 4, 2.
Dalton, about 1803; Thomson’s electron, 1897; plum-pudding model, 1904; gold-foil results; Rutherford’s nuclear model, 1911; Bohr, 1913. The electron had to be found before a model could contain it, and the gold-foil result came before the nuclear model it led to.
Go Deeper
2. 15 200 ÷ 16 000 × 100 = 95%. [2 marks: 1 working, 1 answer]
3. In the plum-pudding model the positive charge is spread thinly, so nothing could turn a fast alpha particle around; it predicted that none would bounce back. Even one bounce-back breaks that prediction. The bounce-backs show that the centre of the atom holds something very small, dense and positive: the nucleus. [3 marks: 1 prediction, 1 result contradicts it, 1 small dense positive nucleus]
Show What You Know
4. Marking guide (points expected, any two changes): Dalton → Thomson: a solid, indivisible atom could not explain electricity; cathode rays gave identical tiny negative particles (electrons) from every metal, so atoms have parts, and the plum-pudding model followed. Thomson → Rutherford: a few alpha particles bounced back from gold foil, so the atom has a tiny, dense, positive nucleus and is mostly empty space. Rutherford → Bohr: electrons circling a nucleus should spiral in, and heated elements give only a few sharp colours, so electrons were placed in fixed shells. 2 marks per change (1 evidence, 1 new model); 1 older model useful: it was the best explanation of the evidence at the time (for example, Dalton’s explained fixed ratios in reactions). [5 marks]
Common error to watch for: placing the gold-foil event after Rutherford’s nuclear model. The experiment came first; the nucleus was the only explanation left once the results came in.