OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify which electron configuration box diagram correctly represents an atom of carbon

Practise this question

Question

Multiple choice question 1 asking which diagram shows the correct electron arrangement in a carbon atom. Four options A, B, C, and D show orbital box diagrams representing 1s, 2s, and 2p energy levels with arrows indicating electrons. Below the options is an answer box and the mark allocation [1].
Question text

1 In the diagrams below, each box represents an orbital and each electron is shown as an arrow.

Which diagram shows the correct arrangement of electrons in an atom of carbon?

2p

2s

A energy

1s

2p

2s

B energy

1s

2p

2s

C energy

1s

2p

2s

D energy

1s

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows that the correct answer for question 1 is D, worth 1 mark under AO1.1.

AO

Question Answer Marks Guidance

element

1 D 1 AO1.1

How to answer it

Electron Configurations in Box Notation

📌 What this question tests

This question assesses your understanding of electronic structure, specifically how electrons fill atomic orbitals according to the Aufbau principle, Pauli exclusion principle, and Hund's rule. You must be able to write and interpret electron configurations using box-and-arrow diagrams for ground-state atoms.

Question 1

Identifying the correct electron arrangement for a carbon atom

✅ Correct Answer: D

Carbon has an atomic number of 6, meaning it has 6 electrons. The electron configuration is 1s² 2s² 2p² .

  • 1s orbital: Fully filled with 2 electrons (opposite spins).
  • 2s orbital: Fully filled with 2 electrons (opposite spins).
  • 2p orbitals: Contains 2 electrons placed singly in separate degenerate orbitals with parallel spins (Hund's rule).

💡 Key Knowledge

  • Aufbau Principle: Orbitals lowest in energy are filled first ( 1s before 2s , 2s before 2p ).
  • Pauli Exclusion Principle: An orbital can hold a maximum of two electrons, and they must have opposite spins (represented by opposing vertical arrows).
  • Hund's Rule: Electrons will occupy degenerate orbitals (orbitals of the same energy, like the three 2p orbitals) singly before pairing up, with their spins parallel.

🧠 Exam Technique

  • Count the total number of electrons in the given diagrams first to quickly eliminate incorrect options. Carbon = 6 electrons.
  • Check orbital filling order from bottom to top (energy axis).
  • Inspect degenerate 2p sub-shells carefully to ensure single occupation before pairing.

❌ Common Errors

  • Diagram A: Violates Hund's rule by pairing two electrons in a single 2p orbital while leaving an adjacent 2p orbital empty.
  • Diagram B: Places 3 electrons in the 2p sub-shell, giving a total of 7 electrons (this is nitrogen with an extra electron, or an excited state).
  • Diagram C: Pairs electrons incorrectly in the 2s and 2p sub-shells without filling lower-energy degenerate slots properly.
Mark breakdown: [1 mark] awarded for selecting option D.

Topics

Module 2: Foundations in chemistry · 2.2 Electrons, bonding and structure

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.