OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2017: Question 1

1 mark · Easy difficulty · Multiple Choice

Identify which atom is not an isotope of iodine from a given table of neutron numbers and mass numbers

Practise this question

Question

A multiple choice question asking which atom is not an isotope of iodine, accompanied by a table with options A to D listing the number of neutrons and mass numbers: A (72 neutrons, 125 mass number), B (74 neutrons, 127 mass number), C (75 neutrons, 128 mass number), and D (77 neutrons, 129 mass number). Below the table is an answer box and the mark allocation [1].
Question text

1 Which atom is not an isotope of iodine?

Number of neutrons Mass number

A 72 125

B 74 127

C 75 128

D 77 129

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicating that the correct answer for question 1 is D.

1 D 1

How to answer it

Identifying Isotopes from Atomic Structure Data

What this question tests

This question assesses your fundamental understanding of atomic structure, specifically the definition of isotopes, atomic number (proton number), mass number, and how to calculate the number of neutrons in an atom (Mass Number minus Proton Number).

Question 1

Exam Breakdown

✅ Correct Answer: D

Option D is not an isotope of iodine because the calculated atomic number does not match iodine's fixed proton number (53).

💡 Key Knowledge

  • Iodine's Atomic Number: Iodine (I) always has a proton number of 53 on the Periodic Table.
  • Definition of Isotopes: Atoms of the same element with the same number of protons (atomic number) but different numbers of neutrons (mass numbers).
  • Formula: Protons = Mass Number − Neutrons.

🧠 Exam Technique

Work methodically through each option by subtracting the number of neutrons from the mass number to find the proton (atomic) number for each row.

❌ Common Errors

Students sometimes confuse mass number with the number of neutrons, or forget that an element's identity is strictly defined by its proton number.

📐 Step-by-Step Verification

  1. A: 125 (Mass) − 72 (Neutrons) = 53 protons (Iodine — IsValid)
  2. B: 127 (Mass) − 74 (Neutrons) = 53 protons (Iodine — IsValid)
  3. C: 128 (Mass) − 75 (Neutrons) = 53 protons (Iodine — IsValid)
  4. D: 129 (Mass) − 77 (Neutrons) = 52 protons (Tellurium, not Iodine — Incorrect/Correct Answer)
Mark Scheme Allocation: 1 mark awarded for selecting D.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.