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Date	May 2022	Marks available	2	Reference code	22M.2.sl.TZ2.2
Level	SL	Paper	2	Time zone	TZ2
Command term	Suggest	Question number	2	Adapted from	N/A

Question

Electrons are arranged in energy levels around the nucleus of an atom.

The diagram represents possible electron energy levels in a hydrogen atom.

Explain why the first ionization energy of calcium is greater than that of potassium.

[2]

All models have limitations. Suggest two limitations to this model of the electron energy levels.

[2]

b(i).

Draw an arrow, labelled X, to represent the electron transition for the ionization of a hydrogen atom in the ground state.

[1]

b(ii).

Draw an arrow, labelled Z, to represent the lowest energy electron transition in the visible spectrum.

[1]

b(iii).

Markscheme

increasing number of protons/nuclear charge/Z_eff ✔

«atomic» radius/size decreases
OR
same number of energy levels
OR
similar shielding «by inner electrons» ✔

Any two of:

does not represent sub-levels/orbitals ✔

only applies to atoms with one electron/hydrogen ✔

does not explain why only certain energy levels are allowed ✔

the atom is considered to be isolated ✔

does not take into account the interactions between atoms/molecules/external fields ✔

does not consider the number of electrons the energy level can fit ✔

does not consider probability of finding electron at different positions/OWTTE ✔

Do not accept “does not represent distance «from nucleus»”.

b(i).

upward arrow X AND starting at n = 1 extending to n = ∞ ✔

b(ii).

downward or upward arrow between n = 3 and n = 2 ✔

b(iii).

Examiners report

It was surprising that this question that appears regularly in IB chemistry papers was not better answered. Many candidates only obtained one of the two marks for identifying one factor (often the larger nuclear charge of calcium or that the number of shells was the same for Ca and K). However, a few candidates did write thorough answers reflecting a good understanding of the factors affecting ionization energy. This question had a strong correlation between candidates who scored well and those who had a high score overall. Some candidates did not score any marks by focusing on trends in the Periodic Table without offering an explanation, or by discussing the number of electrons in Ca and K instead of the number of protons.

[N/A]

b(i).

Only 30% of the candidates drew the correct arrow on the diagram representing the ionization of hydrogen. A few candidates missed the mark by having the arrow pointing downwards. The most common incorrect answer was a transition between n=1 and n=2.

b(ii).

[N/A]

b(iii).

Syllabus sections

Nature of Science

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