Year 8 Science · Unit 2 · Lesson 11
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Learning Goals
Match each question to a model
For each question, choose the simplest model that can answer it. Write its letter in the middle column. Each letter is used once.
| Question | Letter | Model |
|---|---|---|
| Why do lithium and potassium fizz in water in a similar way? | A. Dalton: solid, indivisible sphere | |
| Why does a neon sign glow with exactly these colours? | B. Thomson: electrons dotted through a positive sphere | |
| Why do elements always combine in fixed amounts to make a compound? | C. Rutherford: tiny, dense nucleus in mostly empty space | |
| Why did a few alpha particles bounce straight back from gold foil? | D. Bohr: electrons in fixed shells around the nucleus | |
| Where does the negative charge on a rubbed balloon come from? | E. Quantum: electrons as a cloud of likely positions |
1. State one thing Dalton’s model cannot explain, and one thing the Bohr model cannot explain.
Odd one out
In each row, three items belong together and one does not. Circle the odd one out and give your reason.
| Row | Items | Odd one out, and why |
|---|---|---|
| 1 | Things the Bohr model shows: a central nucleus · electrons in shells · the exact path of each electron · electrons outside the nucleus | |
| 2 | Reasons classroom models are simplified: because scientists know nothing more · to make the main idea clear · to leave out detail that is not needed · so they are easy to draw and use | |
| 3 | Limitations: Dalton shows no parts inside the atom · Thomson shows no nucleus · Rutherford does not explain how electrons are arranged · Bohr shows electrons in energy levels |
Read the data
Scenario: a textbook survey
A student recorded which atom diagram each textbook used when it first introduced electrons.
| Type of book | Books | Bohr shells | Electron cloud | Other |
|---|---|---|---|---|
| Year 7–10 science | 20 | 16 | 1 | 3 |
| Year 11–12 chemistry | 12 | 7 | 5 | 0 |
| University chemistry | 10 | 1 | 9 | 0 |
Source: sample survey made up for this worksheet.
2. Calculate the percentage of books of each type that used a Bohr shell diagram. Round to the nearest whole number.
Year 7–10 % Year 11–12 % University %
3. Describe the pattern. A student says this proves school books are wrong, because university books use the electron cloud. Explain why this conclusion is weak.
4. A science centre wants the quantum model for two Year 8 displays “because it is the most accurate”: (1) an atom has a tiny, heavy centre; (2) why elements in one column of the periodic table react alike. Recommend a model for each display, with a strength and a limitation, and judge the plan.
Match each question to a model
Why do lithium and potassium fizz in water in a similar way? → D; Why does a neon sign glow with exactly these colours? → E; Why do elements always combine in fixed amounts to make a compound? → A; Why did a few alpha particles bounce straight back from gold foil? → C; Where does the negative charge on a rubbed balloon come from? → B.
1. Dalton (any one): electricity; that atoms contain electrons; any structure inside the atom. Bohr (any one): the exact behaviour of electrons; the colours of light given off by complex atoms. [2 marks: 1 each]
Odd one out
Row 1: the exact path of each electron; the Bohr model does not show real electron paths (this is a limitation), while the other three are shown. Row 2: “because scientists know nothing more”; models are simplified on purpose, not because nothing else is known. Row 3: Bohr shows electrons in energy levels; this is a strength, the other three are limitations.
Read the data
2. Year 7–10: 16 ÷ 20 × 100 = 80%. Year 11–12: 7 ÷ 12 × 100 = 58.3, so 58%. University: 1 ÷ 10 × 100 = 10%. [3 marks: 1 each]
3. Pattern: the higher the level, the smaller the share of Bohr diagrams (80%, 58%, 10%) and the more electron-cloud diagrams. Weak because the books have different purposes: school books need a simple model that shows the nucleus and shells clearly; university courses need the exact electron behaviour of the quantum model. Simplified is not wrong. (The survey is also small and made up.) [4 marks: 1 pattern with data, 1 different purposes, 1 simplified is not wrong, 1 judgement]
Show What You Know
4. Marking guide (points expected): Display 1: Rutherford’s nuclear model, the simplest model showing a tiny, dense nucleus with mostly empty space around it (1); limitation: does not explain how electrons are arranged (1). Display 2: the Bohr model, because elements in one column have the same number of outer-shell electrons (1); limitation: electrons do not move on fixed rings (1). Plan: weak; the quantum model is accurate but mathematically complex, and its clouds would hide the simple idea each display needs (1); the best model is the simplest one that answers the question accurately (1). [6 marks]
Common error to watch for: in the match, choosing Bohr for every question because it is the most familiar. Ask which is the simplest model that still answers the question.