Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 1

3 marks · Medium difficulty · Short Open Response

Identify the electronic configuration of the arsenide ion and explain the meaning of opposing and parallel spins in electron box diagrams for phosphorus.

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Question

Question 1 asks about the electronic structure of Group 5 elements. Part (a) is a multiple-choice question asking for the electronic configuration of the arsenide ion, As3-, with options A, B, C, and D showing various electron configurations. Part (b) provides the electronic configuration of a phosphorus atom as 1s2 2s2 2p6 3s2 3p3 and shows a box-and-arrow diagram for its orbitals. Subpart (b)(i) asks to state what is meant by the two arrows in the first box. Subpart (b)(ii) asks to state why the arrows are all pointing in the same direction in the 3p boxes.
Question text

1 This question is about the electronic structure of some Group 5 elements.

(a) Which is the electronic configuration of the arsenide ion, As3–?

(1)

A 1s22s22p63s23p63d104s2

B 1s22s22p63s23p63d104s24p3

C 1s22s22p63s23p63d104s24p6

D 1s22s22p63s23p63d104s24p34d3

(b) The electronic configuration of a phosphorus atom can be written

1s22s22p63s23p3

An alternative way to express the electronic configuration is

1s2 2s2 2p6 3s2 3p3

(i) State what is meant by the two arrows in the first box.

(1)

(ii) State why the arrows are all pointing in the same direction in the 3p boxes.

(1)

(Total for Question 1 = 3 marks)

Mark scheme

Show the mark scheme The mark scheme for Question 1 indicates that the correct answer for 1(a) is C, explaining why options A, B, and D are incorrect. For 1(b)(i), it accepts answers referring to electrons with opposite spin in the same orbital. For 1(b)(ii), it accepts answers stating that electrons have parallel or the same spin in different orbitals according to Hund's rule.

How to answer it

Electronic Structure of Group 5 Elements

What this question tests

This question assesses your understanding of electron configurations, including negative ions, subshell filling order (Aufbau principle), and spin-pairing within orbitals (Hund's rule and Pauli exclusion principle).

Question 1(a): Electronic Configuration of an Ion

Determine the electronic configuration of the arsenide ion, As³⁻

✅ Correct Answer

C: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶

Awarded 1 mark for selecting C.

💡 Key Knowledge

  • Arsenic (As) has an atomic number of 33. A neutral atom has 33 electrons.
  • The As³⁻ ion has gained 3 electrons, giving it a total of 33 + 3 = 36 electrons.
  • The configuration of 36 electrons fills up to the 4p subshell completely ( 4p⁶ ), matching the noble gas krypton.

❌ Common Errors

  • Option A ( 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² ): This represents the configuration of ³³As³⁺ (lost 3 electrons, total 30).
  • Option B ( 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p³ ): This is the ground-state configuration of a neutral arsenic atom ( ³³As ) with 33 electrons.
  • Option D ( ... 4s² 4p³ 4d³ ): Violates the Aufbau filling order by skipping subshells incorrectly.

Question 1(b)(i): Spin in Orbitals

State what is meant by the two arrows in the first box ( 1s² ).

✅ Correct Answer

The arrows represent electrons with opposite spin (or two electrons in the same orbital with opposite spins).

Awarded 1 mark for referencing 'opposite spin'.

🧠 Exam Technique

Examiners are very strict with terminology here. You must explicitly mention spin. Vague answers referring only to "moving in different directions" or just stating "two electrons" without linking to their quantum spin property will not gain credit.

❌ Common Errors

  • Stating just "two electrons" without mentioning spin.
  • Stating "moving in opposite directions" instead of referencing spin property.

Question 1(b)(ii): Hund's Rule

State why the arrows are all pointing in the same direction in the 3p boxes.

✅ Correct Answer

They represent (three) electrons with parallel / same spin because the electrons are in different orbitals (Hund's rule).

Awarded 1 mark for referencing parallel/same spin in separate orbitals.

💡 Key Knowledge

Hund's rule states that degenerate orbitals (orbitals of the same energy, like the three 2p or 3p orbitals) are each occupied singly before any orbital is doubly occupied, and all singly occupied orbitals have electrons with parallel spins to minimize electron-electron repulsion.

🧠 Exam Technique

Make sure your answer connects the same direction of spin to the fact that the electrons occupy separate (different) orbitals. Simply writing "direction of spin" without specifying that they are the same direction is a common trap that loses the mark.

Topics

Physical Chemistry · Topic 1: Atomic Structure and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.