48 exam-style questions on Electron configurations — topic S1.3 of Structure 1: Models of the particulate nature of matter in the IB Chemistry syllabus — with answers and explanations. Samples below; the full set is free with an account.
What is the full ground-state electron configuration of an iron atom (Z = 26)?
Answer: 1s²2s²2p⁶3s²3p⁶4s²3d⁶
The 4s sub-shell fills before 3d, giving [Ar]4s²3d⁶ for the 26 electrons of iron.
State the maximum number of electrons that can occupy the third principal energy level (n = 3), including the d sub-level.
Answer: 18
Maximum electrons in a shell = 2n²; for n = 3 this is 2 × 9 = 18 (3s + 3p + 3d = 2 + 6 + 10).
State the full ground-state electron configuration of a calcium atom (Z = 20).
Answer: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
The 4s sub-level fills before 3d, so calcium's 20 electrons give 1s² 2s² 2p⁶ 3s² 3p⁶ 4s².
Determine the abbreviated (noble-gas core) electron configuration of a potassium atom (Z = 19).
Answer: [Ar] 4s¹
Argon (Z = 18) is the preceding noble gas, and the nineteenth electron enters the 4s sub-level, giving [Ar] 4s¹.
Explain the general trend in first ionisation energy across period 3 from sodium to argon.
Answer: It increases due to increasing nuclear charge and decreasing atomic radius
Across a period the nuclear charge rises while electrons enter the same shell, so the radius shrinks and the outer electron is held more tightly, raising ionisation energy overall.
State the correct order in which sub-levels are filled according to the aufbau principle.
Answer: 3s → 3p → 4s → 3d
The 4s sub-level is lower in energy than 3d and so fills first; the correct order is 3s, 3p, 4s, then 3d.
That was 6 of 48 Electron configurations questions.
Practise all 48 freeStateAO1 · 1–2 marks
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48 Electron configurations questions — free with an account. Spaced repetition, streaks and full exam simulations included.
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