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AS · Practice questions · Shells, orbitals and electron configuration

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Six original Cambridge-style questions on electron configuration: atoms, ions, unpaired electrons, the two d-block exceptions, and identifying an element from its configuration.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

One order, two exceptions.

01
Recall
[3 marks]

Write the full electron configurations of phosphorus (Z = 15), calcium (Z = 20) and nickel (Z = 28).

  • P: 1s22s22p63s23p3. ✓
  • Ca: 1s22s22p63s23p64s2. ✓
  • Ni: 1s22s22p63s23p63d84s2. ✓
02
Explain
[3 marks]

Describe how the 2p electrons are arranged in the orbitals of a nitrogen atom and of an oxygen atom, and state the rule you have used.

  • Nitrogen, 2p3: one electron in each of the three 2p orbitals, all with parallel spins, so three unpaired electrons. ✓
  • Oxygen, 2p4: the fourth electron pairs up in one of the 2p orbitals, leaving two unpaired electrons. ✓
  • Hund's rule: electrons occupy orbitals of equal energy singly, with parallel spins, before any orbital is doubly occupied. ✓
03
Explain
[2 marks]

Write the electron configuration of copper, and explain why it is not [Ar] 3d9 4s2.

  • Cu: [Ar] 3d10 4s1. ✓
  • A completely filled 3d sub-shell gives extra stability, so one 4s electron moves into 3d; the 4s and 3d sub-shells are very close in energy, which makes this possible. ✓
04
Analysis
[3 marks]

Write the electron configurations of the ions Cu2+, Br− and Ti3+.

  • Cu2+: copper is [Ar] 3d10 4s1; remove the 4s electron then one 3d electron, giving [Ar] 3d9. ✓
  • Br−: bromine (35) is [Ar] 3d10 4s2 4p5; add one electron to 4p, giving [Ar] 3d10 4s2 4p6. ✓
  • Ti3+: titanium (22) is [Ar] 3d2 4s2; remove both 4s electrons then one 3d, giving [Ar] 3d1. ✓
05
Explain
[2 marks]

Explain why the 4s sub-shell is filled before the 3d sub-shell in potassium and calcium, yet the 4s electrons are the first to be removed when iron forms Fe2+.

  • In potassium and calcium the empty 4s sub-shell is slightly lower in energy than 3d, so 4s fills first. ✓
  • Once 3d contains electrons its energy drops below that of 4s, so in iron the 4s electrons are the highest-energy electrons and are removed first. ✓
06
Analysis
[2 marks]

An element has the electron configuration [Ar] 3d10 4s2 4p3. Identify the element and state its group in the Periodic Table.

  • Total electrons = 18 + 10 + 2 + 3 = 33, so Z = 33: arsenic. ✓
  • The outer configuration is 4s2 4p3, five outer electrons, so it is in Group 15. ✓

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