OCR A Level Chemistry A: Electron structure (H432) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover 2.2.1 Electron structure from Module 2 (Foundations in chemistry) of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. Between them they test every statement in 2.2.1(a) to (d), including electrons in box diagrams and the HSW link on refining models. H432 is the full A level with no tiers.

Revise first with the electron structure study guide and the electron structure revision notes. The H432 course hub and the topic checklist place 2.2.1 within Module 2.

Use atomic numbers given in each question. Write configurations in full sub-shell notation unless a question says otherwise.

Questions

1. State the maximum number of electrons that can occupy:

(a) the third shell [1] (b) the fourth shell. [1]

2. Describe what is meant by an atomic orbital. [2]

3.

(a) Describe the shape of an s-orbital. [1] (b) Describe the shape of a p-orbital and how the orbitals in a 3p sub-shell are arranged. [2]

4. For each sub-shell, state the number of orbitals it contains and the maximum number of electrons it can hold: 4s, 3p and 3d. [3]

5. Arrange these sub-shells in order of increasing energy: 3d, 4p, 3p, 4s, 2p. [2]

6. Iron has atomic number 26.

(a) Write the full electron configuration of an iron atom. [1] (b) Draw an electrons in box diagram for the 3d and 4s sub-shells of an iron atom. [2] (c) State the number of unpaired electrons in an iron atom. [1]

7. Write the full electron configuration of each ion.

(a) Ca²⁺ (Z = 20) [1] (b) Br⁻ (Z = 35) [1] (c) Ga⁺ (Z = 31) [1]

8. Consider these species: Ar, Cl⁻, K⁺, Na⁺, S²⁻, Ne, Ca²⁺, O²⁻.

(a) Identify every species with the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶. [2] (b) State the term used for species that share the same electron configuration. [1]

9. Chromium (Z = 24) and copper (Z = 29) do not follow the simple filling order.

(a) For chromium, write the configuration that the filling order predicts and the actual configuration. [2] (b) Draw an electrons in box diagram for the 3d and 4s sub-shells in the actual configuration of chromium. [2] (c) State the number of unpaired electrons in a chromium atom. [1] (d) Write the actual electron configuration of copper and suggest why it differs from the predicted one. [2]

10. Element X is in Period 4. An atom of X has exactly three unpaired electrons.

(a) Identify the three Period 4 elements whose atoms have exactly three unpaired electrons. Give the outer sub-shell configuration that shows the unpaired electrons in each case. [3] (b) X is a p-block element. Identify X and write the full electron configuration of the ion X³⁻. [2] (c) State the number of electrons in X³⁻ and name the noble gas atom that has the same configuration. [2] (d) State how many orbitals are occupied in X³⁻. [1]

11. Germanium has atomic number 32.

(a) Write the full electron configuration of a germanium atom. [1] (b) State the total number of electrons in s-orbitals. [1] (c) State the total number of electrons in p-orbitals. [1] (d) State the number of orbitals that are completely filled and the number that are half-filled. [2] (e) Explain why the two 4p electrons are placed in separate orbitals with parallel spins. [2]

12. A student uses a simple shell model and predicts that calcium (Z = 20) has the configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d². Explain, in terms of sub-shell energies, why this is wrong, and give the correct configuration. [4]

Answers

1. (a) 18 [1] (b) 32 (4s, 4p, 4d and 4f together) [1] Examiner insight: Answers of 18 for the fourth shell come from counting only s, p and d; recall 2n² rather than adding up the sub-shells you have studied.

2. A region around the nucleus [1] that can hold up to two electrons, with opposite spins [1]. Examiner insight: “Opposite spins” is part of the definition the specification uses; a description that stops at “where an electron is found” leaves the second point unmade.

3. (a) Spherical, centred on the nucleus [1] (b) Dumbbell shaped, with two lobes on opposite sides of the nucleus [1]; there are three p-orbitals at right angles to each other (along the x, y and z axes) [1] Examiner insight: A sketch can support the answer, but the words “three” and “at right angles” are what show you know the arrangement, so state them even if you draw.

4. 4s: 1 orbital, 2 electrons [1]; 3p: 3 orbitals, 6 electrons [1]; 3d: 5 orbitals, 10 electrons [1] Examiner insight: Each sub-shell needs both numbers; giving electrons in place of orbitals (6 orbitals for 3p) is a common slip that loses the point for that row.

5. 4s placed below 3d [1]; full order 2p < 3p < 4s < 3d < 4p [1] Examiner insight: Write the order with “<” signs or say “lowest first” so there is no doubt about which end of the list is lowest in energy.

6. (a) 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s² (26 electrons) [1] (b)

3d  [↑↓][↑ ][↑ ][↑ ][↑ ]
4s  [↑↓]

Five 3d boxes with one pair and four single electrons of the same spin [1]; 4s box holding a pair with opposite spins [1] (c) 4 [1] Examiner insight: In the box diagram, the four single 3d electrons must all point the same way; mixing arrow directions among unpaired electrons does not show singly-before-pairing correctly.

7. (a) 18 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ [1] (b) 36 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ [1] (c) 30 electrons; the single 4p electron is removed: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² [1] Examiner insight: For Ga⁺, removing a 3d electron and keeping 4p¹ gives the right total but the wrong configuration; the electron lost comes from the outermost sub-shell.

8. (a) Ar, Cl⁻ and K⁺ [1]; S²⁻ and Ca²⁺, with no 10-electron species (Na⁺, Ne, O²⁻) included [1] (b) Isoelectronic [1] Examiner insight: Listing extra species alongside the correct ones undermines the answer, so check each one by working out Z minus charge before you include it.

9. (a) Predicted: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁴ 4s² [1]; actual: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹ [1] (b)

3d  [↑ ][↑ ][↑ ][↑ ][↑ ]
4s  [↑ ]

One electron in each of the five 3d boxes [1]; a single electron in 4s, all six with parallel spins [1] (c) 6 [1] (d) 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s¹ [1]; a completely filled 3d sub-shell is a more stable (lower energy) arrangement than 3d⁹ 4s² [1] Examiner insight: When asked for the actual configuration, an answer of 3d⁴ 4s² for chromium is treated as wrong even though it follows the filling rule; learn these two atoms as facts.

10. (a) Vanadium, 3d³ 4s² [1]; cobalt, 3d⁷ 4s² (two paired and three single d electrons) [1]; arsenic, 4s² 4p³ [1] (b) X is arsenic (Z = 33), the only p-block element of the three [1]; the three extra electrons complete 4p, so As³⁻ is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ [1] (c) 33 + 3 = 36 electrons [1]; krypton [1] (d) 1 + 1 + 3 + 1 + 3 + 1 + 5 + 3 = 18 orbitals [1] Examiner insight: For cobalt, show why seven d electrons leave three unpaired (five singly, then two pair up); a bare element name with no supporting configuration is harder to credit if the question asks you to show it.

11. (a) 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p² [1] (b) 1s² + 2s² + 3s² + 4s² = 8 [1] (c) 2p⁶ + 3p⁶ + 4p² = 14 [1] (d) 15 completely filled (1s, 2s, three 2p, 3s, three 3p, five 3d, 4s) [1]; 2 half-filled (two 4p orbitals) [1] (e) The three 4p orbitals have the same energy, and electrons repel each other [1], so they occupy separate orbitals singly, with parallel spins, before any pairing [1] Examiner insight: In counting questions, write out the sub-shells you are adding; a single number with no working gives no way to see where a slip happened.

12. After 3p is full (at 18 electrons), the next orbital in energy order is 4s, not 3d [1]. 4s has a lower energy than 3d [1], and electrons go into the lowest-energy orbital available [1]. Correct configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² [1] Examiner insight: “4s fills first” on its own describes the order; the explanation needs the energy comparison between 4s and 3d and the lowest-energy-first rule.

Where marks are usually lost

• Giving 18 as the capacity of the fourth shell.
• Defining an orbital without “up to two electrons” or without “opposite spins”.
• Confusing the number of orbitals in a sub-shell with the number of electrons it holds.
• Box diagrams where electrons pair in one orbital while another orbital of the same energy is empty.
• Unpaired electrons drawn with mixed spin directions.
• Missing the chromium and copper exceptions when the question asks for the actual configuration.
• Removing 3d electrons from Period 4 p-block atoms when forming cations.
• Totals that do not match Z adjusted for charge, especially for anions such as As³⁻ and Br⁻.
• Counting unpaired electrons from the superscript alone (4p⁴ has two unpaired electrons, not four).

Next steps

• Electron structure revision notes
• Electron structure study guide
• H432 course hub
• H432 topic checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, published by OCR. Section 2.2.1 Electron structure.