IB Chemistry HL S3.1 The periodic table: classification of elements questions

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Question 1

Multiple choiceEasy1 mark

A data set gives atomic radii for four consecutive period 3 elements: Na, 186 pm; Mg, 160 pm; Al, 143 pm; Si, 118 pm. What is the mean decrease in atomic radius per step from Na to Si, to the nearest whole picometre?

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Answer: B. The mean decrease per step is the total decrease divided by the number of intervals: (186 − 118) pm ÷ 3 = 22.7 pm, which rounds to 23 pm.

A: divides the total decrease by the four elements rather than the three intervals, giving 17 pm. C: divides by two instead of three intervals, giving 34 pm. D: gives the total decrease, 68 pm, rather than the mean decrease per step.

Remember: four consecutive elements have three intervals between them.

Question 2

Multiple choiceMedium1 mark

In a simulation, bromine water is added separately to aqueous potassium chloride and potassium iodide. Which prediction follows from the Group 17 trend in non-metallic character?

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Answer: A. Non-metallic character and halogen reactivity decrease down Group 17. Thus, Cl₂ is more reactive than Br₂, while Br₂ is more reactive than I₂. Bromine cannot displace Cl⁻ but can displace I⁻, forming Br⁻ and I₂.

B: Reversing the reactivity order makes Br₂ appear able to displace Cl⁻ but not I⁻. C: Correctly predicts iodide displacement but wrongly extends it to chloride, whose halogen is more reactive. D: Fails to apply the reactivity comparison to iodide; Br₂ displaces I⁻. Remember: A halogen displaces the halide of a less reactive halogen.

Question 3

Paper 2Hard8 marks

Cobalt(II) occurs in differently coloured complexes. A student records the appearance and wavelength of maximum absorption for two solutions.

Complex in solutionAppearanceWavelength of maximum absorption / nm
[Co(H₂O)₆]²⁺pink535
[CoCl₄]²⁻ in chloride-rich solutionblue610
(a)

State what happens to an electron when visible light is absorbed by a transition-element complex.

[1]
(b)

Use the colour wheel in the data booklet to identify the colour of light absorbed by each solution.

[2]
(c)

A student argues: “Because the solution looks blue, its complex absorbs higher-frequency light than the pink solution.” Assess this claim quantitatively, using the data.

[5]

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