WJEC A-Level Chemistry AS Unit 1, June 2025: Question 5

1 mark · Medium difficulty · Short Answer

State the total number of occupied orbitals in a nitrogen atom.

Practise this question

Question

Question 5 asks: 'State the total number of orbitals that are occupied in a nitrogen atom.' followed by an answer line and indicated as worth 1 mark.

Mark scheme

Show the mark scheme Mark scheme for question 5 showing the accepted answer '5' with 1 mark awarded in the total column.

How to answer it

Determining Occupied Atomic Orbitals in Nitrogen

📌 What this question tests

This question assesses your understanding of atomic structure and electronic configuration under WJEC AS Unit 1. Specifically, it tests your ability to distinguish between principal energy levels, subshells, and individual atomic orbitals, alongside the application of Hund's rule of maximum multiplicity.

Question 5 • Total: [1 Mark]

Question 5: Total Number of Occupied Orbitals

Topic: Electronic Configuration & Orbital Filling

✅ Correct Answer

5

Mark breakdown:
• [1 mark] for stating 5 (or writing out five distinct occupied orbitals).

📐 Step-by-Step Breakdown

  1. Find atomic number (Z): Nitrogen has Z = 7 , so a neutral nitrogen atom has 7 electrons.
  2. Write subshell configuration:
    1s² 2s² 2p³
  3. Break down by orbital:
    • 1s subshell = 1 orbital (containing 2 paired electrons)
    • 2s subshell = 1 orbital (containing 2 paired electrons)
    • 2p subshell = 3 orbitals ( 2px , 2py , 2pz )
  4. Apply Hund's Rule: The 3 electrons in the 2p subshell occupy each degenerate orbital singly: 1 electron in 2px , 1 in 2py , and 1 in 2pz .
  5. Sum the occupied orbitals:
    1 (from 1s) + 1 (from 2s) + 3 (from 2p) = 5 occupied orbitals.

💡 Key Knowledge

  • Orbital Definition: A region around the nucleus that can hold up to 2 electrons with opposite spins.
  • Subshell capacities:
    • s subshell: 1 orbital (max 2 e⁻)
    • p subshell: 3 orbitals (max 6 e⁻)
    • d subshell: 5 orbitals (max 10 e⁻)
  • "Occupied" vs "Fully Occupied": An orbital is considered occupied if it contains at least one electron. It does not need to be full.
  • Hund's Rule: Electrons occupy degenerate (equal energy) orbitals singly before pairing up.
Examiner Visualization – Electrons in Boxes:
If drawn as spin-box diagrams:
• [ ⥮ ] (1s orbital – occupied)
• [ ⥮ ] (2s orbital – occupied)
• [ ⥿ ][ ⥿ ][ ⥿ ] (three separate 2p orbitals – all occupied singly)
Total boxes with at least one arrow = 5 boxes.

❌ Common Errors

  • Confusing orbitals with electrons: Writing 7 (the total number of electrons in N).
  • Confusing orbitals with subshells: Writing 3 (the number of subshells: 1s, 2s, 2p).
  • Confusing orbitals with principal energy levels: Writing 2 (shells n = 1 and n = 2).
  • Ignoring Hund's Rule: Assuming pairing in the 2p subshell (e.g., thinking 3 electrons would occupy only 2 orbitals: [⥮][⥿][ ]).

🧠 Exam Technique

  • Read the exact term carefully: Circle the keyword: is it asking for shells, subshells, orbitals, or electrons?
  • Sketch rough boxes: When asked about orbital numbers, quickly sketch "electrons in boxes" on rough paper to avoid mental counting errors.
  • Single mark efficiency: This is a 1-mark question in Section A. Simply writing 5 scores the mark immediately—no lengthy explanation is required.

Topics

Physical Chemistry · 1.2 Basic ideas about atoms

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.