Year 8 Science · Unit 2 · Lesson 10
Challenge Worksheet
Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Thomson’s model explained why an atom has no overall charge: its positive and negative charges balance.
Correct it:
2. The gold-foil results showed that an atom is mostly solid matter.
Correct it:
3. Dalton’s model could explain how a metal wire carries an electric current.
Correct it:
4. The plum-pudding model predicted that no alpha particles would bounce back from gold foil.
Correct it:
5. Bohr’s model came after Rutherford’s and kept the nucleus at its centre.
Correct it:
1. Which observation ended the solid-sphere model, and which one ended the plum-pudding model?
2. In early experiments with thin metal foils, roughly 1 alpha particle in every 8000 bounced back. An observer counts 30 bounce-backs. About how many alpha particles were fired? About how many did not bounce back?
Fired particles
3. Use the 1 in 8000 figure to explain why the nucleus must take up only a very small part of each atom, and why it must be dense and positive.
| Light source | What is seen through the spectroscope |
|---|---|
| Filament light globe | a smooth band of every colour, red to violet |
| Hydrogen gas tube | four sharp lines: red, blue-green, blue-violet, violet |
| Sodium street lamp | one bright yellow line (really two, very close) |
| Neon sign | many separate sharp lines, mostly red and orange |
A spectroscope splits light into its colours. Data: standard observations.
4. Which sources in Table 1 give out only a few sharp colours, and what do they have in common? Explain how Bohr’s shells account for the hydrogen result, and why Rutherford’s model, with no shells, could not.
Someone claims
“Science moves forward by proving old ideas wrong and throwing them away. Rutherford proved Thomson wrong, and that is all that matters.”
5. Evaluate the claim. Use at least two of the models from this lesson as evidence, and say what each later model kept, what it changed, and why.
True or false
1. True. 2. False: the atom is mostly empty space, with a tiny, dense, positive nucleus. 3. False: Dalton’s solid atom had no parts that could move, so it could not explain electric current. 4. True. 5. True.
1. Solid sphere: cathode rays were made of identical tiny negative particles (electrons) whatever metal was used, so atoms have parts. Plum pudding: a few alpha particles bounced back from gold foil, which it could not explain. [2 marks]
Your Turn
2. 30 × 8000 = 240 000 fired; 240 000 − 30 = 239 970 did not bounce back. [2 marks]
3. Only about 1 in 8000 particles hit something that turned it back, so that part must be a very small target; nearly all of the atom lets particles through. To turn back a fast, heavy alpha particle it must hold most of the atom’s mass (dense), and be positive to repel the positive alpha particles. [3 marks: 1 small target, 1 dense, 1 positive]
Go Deeper
4. Hydrogen gas, the sodium lamp and the neon sign: each is a single glowing element (a gas or vapour), not a hot solid filament. In Bohr’s model electrons can only be in particular shells, so they can only move between the shells that exist; each move gives out one particular colour, so hydrogen gives a few sharp lines, not every colour. Rutherford’s model had no fixed shells, so it gave no reason for only certain colours. [4 marks: 1 sources and what they share, 1 fixed shells, 1 only certain colours, 1 Rutherford’s limit]
Show What You Know
5. Marking guide (points expected): 1 fair part: the gold-foil results showed the plum-pudding model was wrong about where the positive charge is; 1 misleading: later models keep what the evidence supports; 1 Thomson → Rutherford: electrons kept, spread-out positive charge replaced by a nucleus because of the bounce-backs; 1 Rutherford → Bohr: nucleus kept, shells added to explain stable atoms and sharp colours (or Dalton → Thomson: atoms kept, electrons added); 1 each model was the best fit to the evidence of its time, and Thomson’s clear prediction made the gold-foil test possible; 1 better statement, e.g. “Models change step by step when new evidence cannot be explained, keeping what still fits.” [6 marks]
Common error to watch for: in Q2, answering 30 ÷ 8000. Students should check that far more particles were fired than bounced back.