AQA A-Level Chemistry AS Paper 1, November 2021: Question 21

1 mark · Easy difficulty · Multiple Choice

Identify which atom has one more proton and two more neutrons than 31P with subscript 15.

Practise this question

Question

Multiple choice question 21 asking which atom has one more proton and two more neutrons than 31/15 P, with four options A (33/16 P), B (34/16 P), C (33/16 S), and D (34/16 S).
Question text

21 Which atom has one more proton and two more neutrons than 31P ?

[1 mark]

A 33P

B 34P

C 33

16S

D 34

16S

Mark scheme

Show the mark scheme Mark scheme showing the correct answer is D, corresponding to sulfur-34.

21 D 1 34

16S

How to answer it

Determining Subatomic Particles in Nuclides

What this question tests

This question assesses your fundamental understanding of atomic structure, specifically how to interpret atomic number (proton number) and mass number (nucleon number) in nuclide notation (AZX format), and how changes to protons and neutrons affect element identity and mass.

Question 21

Identifying an Atom from Protons and Neutrons

✅ Correct Answer

D ( ³⁴₁₆S )

Awarded 1 mark for selecting option D.

💡 Key Knowledge

  • Atomic Number (Z): The bottom number in nuclide notation represents the number of protons, which defines the element.
  • Mass Number (A): The top number represents the total number of protons + neutrons (nucleons).
  • Calculating Neutrons: Neutrons = Mass Number (A) − Atomic Number (Z).

🧠 Exam Technique

Break the question down step-by-step on your rough paper before looking at the options:

  1. Deconstruct the starting atom ( ³¹₁₅P ).
  2. Apply the specified changes mathematically.
  3. Match the final proton and nucleon totals to the periodic table / options.

❌ Common Errors

  • Confusing mass number changes with neutron changes (forgetting that adding a proton changes the element itself).
  • Adding the extra particles directly to the mass number without checking the new atomic number (e.g., choosing option B).

📐 Step-by-Step Breakdown

  1. Analyze the starting atom ( ³¹₁₅P ):
    • Protons (bottom number) = 15
    • Mass number (top number) = 31
    • Neutrons = 31 − 15 = 16
  2. Apply the given modifications:
    • New number of protons = 15 + 1 = 16 (Element with atomic number 16 is Sulfur, S).
    • New number of neutrons = 16 + 2 = 18.
  3. Calculate the new mass number:
    • New Mass Number = (New protons) + (New neutrons) = 16 + 18 = 34.
    • Alternatively: Add the total added particles (1 proton + 2 neutrons = 3 added nucleons) to the original mass number (31 + 3 = 34).
  4. Formulate the final nuclide: ³⁴₁₆S , which corresponds to option D.

Topics

Physical Chemistry · 3.1.1 Atomic Structure

Question and mark scheme from the AQA A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.