Year 8 Science · Unit 2 · Lesson 10
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Learning Goals
Match each observation to its conclusion
Write the matching letter in the middle column. Each letter is used once.
| Observation | Letter | What scientists concluded |
|---|---|---|
| Elements always combine in fixed whole-number ratios. | A. Most of the atom is empty space. | |
| Cathode rays contain the same tiny negative particles, whatever metal is used. | B. The positive charge and nearly all the mass are packed into a tiny nucleus. | |
| Most alpha particles pass straight through thin gold foil. | C. Matter is made of atoms that join in fixed amounts. | |
| A very small number of alpha particles bounce back from gold foil. | D. Electrons can only be in particular shells (energy levels). | |
| A heated element gives out only a few sharp colours of light. | E. Every atom contains electrons, so atoms have smaller parts. |
1. Conclusions A and B together describe one model of the atom. Name the model and the scientist who proposed it.
Scenario: the cathode-ray tube
A cathode-ray tube is a sealed glass tube with the air pumped out and a high voltage across it. Thomson made the negative end (the cathode) from different metals and passed the ray between two charged plates.
| Cathode metal | Ray bends towards | Charge of the particles | Mass of one particle compared with a hydrogen atom |
|---|---|---|---|
| Copper | the positive plate | about 1/1800 | |
| Aluminium | the positive plate | about 1/1800 | |
| Iron | the positive plate | about 1/1800 | |
| Platinum | the positive plate | about 1/1800 |
Source: sample results based on Thomson’s findings (1897); the mass values use the modern figure.
2. Complete the charge column. Explain how the bending of the ray tells you the charge, and what the mass column tells you about the size of these particles.
3. Every metal gave exactly the same particle. Explain why this meant Dalton’s model of a solid atom with no parts had to change.
Cause and effect: a prediction that failed
Complete each link of the chain in one or two sentences.
| Link | Your answer |
|---|---|
| What the plum-pudding model says about positive charge | |
| So it predicts that alpha particles fired at gold foil will… | |
| But the experiment showed that… | |
| So the new model must have… |
4. Rutherford’s nuclear model had a problem of its own, and Bohr’s model (1913) was proposed to fix it. Explain the problem, Bohr’s answer, and one observation his model could explain. Then explain why Bohr’s model is still used in Year 8 even though it is not the final model of the atom.
Match each observation to its conclusion
Elements always combine in fixed whole-number ratios. → C; Cathode rays contain the same tiny negative particles, whatever metal is used. → E; Most alpha particles pass straight through thin gold foil. → A; A very small number of alpha particles bounce back from gold foil. → B; A heated element gives out only a few sharp colours of light. → D.
1. The nuclear model, proposed by Rutherford (1911). [2 marks]
Your Turn
2. Charge: negative in every row. The ray bends towards the positive plate, and opposite charges attract, so the particles are negative. Each particle has about 1/1800 of the mass of a hydrogen atom (the lightest atom), so the particles are far smaller than any atom. [3 marks: 1 column, 1 bending reason, 1 lighter than any atom]
3. A particle that comes out the same from every metal must be part of every atom. So atoms contain smaller parts (electrons) and are not solid and indivisible, which Dalton’s model said they were. The model had to be rebuilt with electrons inside, which became the plum-pudding model. [3 marks: 1 part of every atom, 1 contradicts indivisible, 1 new model]
Cause and effect: a prediction that failed
Positive charge is spread thinly through the whole atom. → So every alpha particle should pass through, with at most a slight change of direction. → But most went straight through and a very few bounced back. → So the new model must have its positive charge and almost all its mass in a tiny, dense nucleus, with the rest mostly empty space.
Show What You Know
4. Marking guide (points expected): 1 on what was known about charges, a negative electron circling a positive nucleus should lose energy and spiral into it; 1 but atoms are stable, so this does not happen; 1 Bohr: electrons can only be in particular shells and are stable while they stay in one; 1 this explains why a heated element gives out only a few sharp colours, because electrons can only move between the shells that exist; 1 still used because it gives a clear, manageable picture of a nucleus with electrons in shells; 1 limit: electrons do not really move in neat circles, and the model has since been improved (the quantum model). [6 marks]
Common error to watch for: matching the gold-foil bounce-backs to “most of the atom is empty space”. The empty space comes from the particles that passed straight through; the bounce-backs point to the small, dense nucleus.