DP Chemistry (first assessment 2025)

Question 22M.2.SL.TZ2.2

Date	May 2022	Marks available	[Maximum mark: 6]	Reference code	22M.2.SL.TZ2.2
Level	SL	Paper	2	Time zone	TZ2
Command term	Draw, Explain, Label, Suggest	Question number	2	Adapted from	N/A

[Maximum mark: 6]

22M.2.SL.TZ2.2

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

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

(a)

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

[2]

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» ✔

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.

(b(i))

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

[2]

Markscheme

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(ii))

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

[1]

Markscheme

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

Examiners report

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(iii))

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

[1]

Markscheme

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

Syllabus sections

Structure 3. Classification of matter » Structure 3.1—The periodic table: Classification of elements » Structure 3.1.3—Periodicity refers to trends in properties of elements across a period and down a group.

Nature of science

Structure 1. Models of the particulate nature of matter » Structure 1.3—Electron configurations » Structure 1.3.6—In an emission spectrum, the limit of convergence at higher frequency corresponds to ionization. Explain the trends and discontinuities in first ionization energy (IE) across a period and down a group. Calculate the value of the first IE from spectral data that gives the wavelength or frequency of the convergence limit.

Structure 1. Models of the particulate nature of matter » Structure 1.3—Electron configurations » Structure 1.3.1—Emission spectra. Emission spectra are produced by atoms emitting photons when electrons in excited states return to lower energy levels. Qualitatively describe the relationship between colour, wavelength, frequency and energy across the electromagnetic spectrum. Distinguish between a continuous and a line spectrum.

Structure 1. Models of the particulate nature of matter » Structure 1.3—Electron configurations » Structure 1.3.2—The line emission spectrum of the hydrogen atom. The line emission spectrum of hydrogen provides evidence for the existence of electrons in discrete energy levels, which converge at higher energies. Describe the emission spectrum of the hydrogen atom, including the relationships between the lines and energy transitions to the first, second and third energy levels.

Question 22M.2.SL.TZ2.2

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