OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2025: Question 1

📌 What This Question Tests

This question assesses fundamental knowledge of atomic structure from Module 2 (Foundations in Chemistry):

Defining an isotope in terms of subatomic particles.
Understanding that an element's chemical identity is uniquely determined by its atomic number (proton number).
Extracting and comparing proton and neutron numbers from tabulated data under exam conditions.

Question 1

Atom	Number of protons	Number of neutrons	Mass number (Protons + Neutrons)	Element Identity
W	17	18	35	Chlorine (³⁵Cl)
X	18	18	36	Argon (³⁶Ar)
Y	19	18	37	Potassium (³⁷K)
Z	18	19	37	Argon (³⁷Ar)

C: X and Z

Mark Scheme Breakdown:
• C = 1 mark.
Any other letter selected = 0 marks.

Isotopes are atoms of the same element with the:
- Same number of protons (same atomic number, Z).
- Different number of neutrons (different mass number, A).
Because they possess identical proton counts (18), both X and Z are atoms of argon.
X has 18 neutrons (mass number 36) while Z has 19 neutrons (mass number 37).

Step 1: Check proton numbers first.
To be the same element, the atoms must have the same number of protons.
• W = 17, X = 18, Y = 19, Z = 18.
Only X and Z share the same number of protons (18).
Step 2: Check neutron numbers to confirm they are isotopes.
• X has 18 neutrons.
• Z has 19 neutrons.
Since 18 ≠ 19, they have different numbers of neutrons and therefore different mass numbers.
Step 3: Match to options.
Pair (X and Z) corresponds directly to option C.

In Section A multiple choice questions, time management is critical:

Scan the "Number of protons" column first. Look for identical numbers.
Only row X and row Z show 18 protons. You can eliminate options A, B, and D in under 10 seconds without needing to calculate anything else.
Always write the final capital letter clearly inside the answer box provided on the exam paper.

Confusing same neutrons with isotopes (Choosing A, B, or W/X/Y):
Notice that W, X, and Y all have 18 neutrons. Students who mistakenly define isotopes as having the "same number of neutrons" fall into this deliberate examiner distractor. Atoms with the same neutron number but different proton numbers are known as isotones, not isotopes.
Confusing same mass number with isotopes (Pairs Y and Z):
Atom Y (19 protons + 18 neutrons = 37) and Atom Z (18 protons + 19 neutrons = 37) have the exact same nucleon/mass number (37). These are isobars, not isotopes, because they have different proton numbers and belong to different elements (potassium and argon).

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