Year 8 Science · Unit 2 · Lesson 12
Challenge Worksheet
Learning Goals
Find the errors
These notes contain exactly three errors. Underline each one and correct it below.
A student’s notes on the chlorine box
The chlorine box shows the symbol Cl, the name chlorine and two numbers, 17 and 35.45. The number 35.45 is the atomic number, so every chlorine atom has 35 protons. The number 17 is the number of neutrons. Because 35.45 is not a whole number, it must have been measured wrongly. Chlorine comes straight after sulfur in the table because it has one more proton.
1. Write the three corrections.
Test a prediction
Scenario: the missing element below silicon
Mendeleev left a gap below silicon and predicted the properties of the element that would fill it, which he called eka-silicon. The element was later discovered and named germanium.
| Property | Mendeleev’s prediction | Germanium (measured) |
|---|---|---|
| Relative atomic mass | 72 | 72.6 |
| Density (g/cm³) | 5.5 | 5.3 |
| Colour | dark grey | grey-white |
| Formula of its oxide | XO₂ | GeO₂ |
| Formula of its chloride | XCl₄ | GeCl₄ |
Source: Mendeleev’s published predictions and modern values for germanium, rounded. X stands for the predicted element.
2. Calculate the percentage difference between prediction and measurement for the relative atomic mass and for the density. Use: percentage difference = difference ÷ measured value × 100. Give each answer to one decimal place.
Mass %
Density %
3. A student says: “The colour and density were not exactly right, so Mendeleev’s prediction failed.” Use all five rows of Table 1 to judge this statement.
Prediction or measurement?
Scenario: the gallium density
Mendeleev predicted a density of about 5.9 g/cm³ for the element below aluminium. Gallium’s discoverer first measured 4.7 g/cm³. Mendeleev suggested the sample was impure; purified gallium measured 5.9 g/cm³.
4. Explain why Mendeleev had good reason to question the first measurement. Then explain why the matter was only settled by the new measurement, not by Mendeleev’s prediction alone.
5. Evaluate this claim, using evidence from this worksheet: “The periodic table is just a list of elements to memorise. Mendeleev was lucky that his guesses about missing elements turned out right.”
Find the errors
1. (i) The atomic number is 17, so every chlorine atom has 17 protons; 35.45 is the relative atomic mass. (ii) 17 is the number of protons, not neutrons; the box does not show the number of neutrons. (iii) 35.45 is not a mistake: it is an average mass for the atoms of chlorine in a normal sample, so it is not a whole number. The last sentence is correct. [3 marks: 1 per correction]
Test a prediction
2. Relative atomic mass: 72.6 − 72 = 0.6; 0.6 ÷ 72.6 × 100 = 0.8%. Density: 5.5 − 5.3 = 0.2; 0.2 ÷ 5.3 × 100 = 3.8%. [4 marks: 1 working and 1 answer for each]
3. Weak statement. Both formulas (GeO₂, GeCl₄) were exactly right, the atomic mass was within 1% and the density within about 4%; only the colour was noticeably different (and it was still grey). A prediction made before the element was found does not need to be exact to show that the pattern in the table works; four of five properties being right or very close is strong support. [3 marks: 1 use of the formulas, 1 use of the percentages, 1 judgement]
Prediction or measurement?
4. Good reason: the prediction came from the pattern of the elements around the gap, a pattern that had worked for other elements, so a big difference (4.7 vs 5.9) pointed to a problem with the sample, such as impurities. Settled by measurement: a prediction is only a claim until tested; the pure sample gave 5.9, so evidence decided it. Had it still measured 4.7, the prediction would have had to change. [4 marks: 1 prediction from pattern, 1 impure sample, 1 evidence decides, 1 prediction could have been wrong]
Show What You Know
5. Marking guide (points expected): 1 the table is arranged by atomic number, with similar elements in the same column, so it shows patterns a list cannot; 1 predictions came from those patterns (the gap below silicon was predicted from its neighbours); 1 germanium matched closely (formulas exact, mass within 1%, density within 4%), which is hard to explain as luck; 1 the gallium prediction was strong enough to question a faulty measurement, and a better measurement confirmed it; 1 students need to use the table (read a box, find an element, compare a column), not memorise it; 1 judgement: the claim is wrong; the table organises, compares and predicts. [6 marks]
Common error to watch for: in Q2, dividing by the predicted value instead of the measured value. Both give a small percentage here, but students should use the formula given.