OCR A-Level Chemistry AS Breadth in chemistry (01), November 2020: Question 21
7 marks · Medium difficulty · Structured Questions
Complete electron shell capacity, state isotope similarities and differences, calculate relative atomic mass from mass spectra, and explain molecular ion peak m/z values for chlorine.
Practise this questionQuestion
Question text
21 This question is about atoms, isotopes and mass spectrometry.
(a) Complete the table to show the number of electrons that can fill the first four shells.
Shell 1st shell 2nd shell 3rd shell 4th shell
Number of electrons
[1]
(b) Most elements contain atoms of different isotopes.
State any differences and similarities between the atomic structures of isotopes of the same
element.
Differences …
Similarities …
[2]
(c) Mass spectrometry can be used to identify the isotopes of chlorine.
Part of the mass spectrum of chlorine is shown below.
75.76
relative
abundance
(%)
24.24
32 33 34 35 36 37 38
m/z
(i) Calculate the relative atomic mass of chlorine.
Give your answer to 2 decimal places.
relative atomic mass = … [2]
(ii) The mass spectrum of chlorine, Cl , also contains three molecular ion, M+, peaks.
One of the M+ peaks has an m /z value of 72.
Suggest why an M+ peak at m /z = 72 is observed and predict the m /z values of the other
two M+ peaks.
Peak at m/z = 72 …
m /z values of the other two M+ peaks …
[2]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
21 (a) Shell 1st shell 2nd shell 3rd shell 4th shell 1 1.1
Electrons 2 8 18 32
Requires all 4 numbers to be correct
(b) Differences: 2 1.1×2 IGNORE different masses/mass numbers
(Different number of) neutrons throughout
(Question asks for atomic structures)
Similarities:
(Same number of) protonsAND electrons ALLOW ‘amount’ for ‘number’
ALLOW ‘electron configuration’ for electrons
(c) (i) FIRST CHECK ANSWER ON THE ANSWER LINE 2 1.2×2
If answer = 35.48 (to 2 DP) award 2 marks
(35 × 75.76) + (37 × 24.24) For 1 mark: ALLOW ECF → to 2 DP if:
OR 35.4848 OR 35.485
100 • %s used with wrong isotopes ONCE
OR
= 35.48 (to 2 DP) • transposed decimal places for ONE %
AND
• calculated Ar is between 35 and 37
(c) (ii) m/z = 72: 35Cl37Cl 2 3.1
OR Contains chlorine-35 AND chlorine-37
m/z values: 70 AND 74 3.2
How to answer it
Atoms, Isotopes and Mass Spectrometry
What this question tests
This question assesses fundamental atomic structure, electron shell capacity rules (2n² formula), the definition of isotopes in terms of subatomic particles, calculating relative atomic mass (Ar) from mass spectrometry data using percentages, and interpreting molecular ion peaks (M⁺) for diatomic molecules like Cl₂.
Part (a) Electron Shell Capacities
Complete the table for the first four electron shells [1 mark]
✅ Correct Answer
- 1st shell: 2
- 2nd shell: 8
- 3rd shell: 18
- 4th shell: 32
💡 Key Knowledge
The maximum number of electrons that can fill a principal shell is given by the formula 2n², where n is the principal quantum number (shell number):
- n = 1 → 2(1)² = 2
- n = 2 → 2(2)² = 8
- n = 3 → 2(3)² = 18
- n = 4 → 2(4)² = 32
❌ Common Errors
Students often lose this mark because all 4 values must be correct. A single incorrect number means 0 marks awarded.
Part (b) Structure of Isotopes
State differences and similarities between atomic structures of isotopes [2 marks]
✅ Correct Answer
- Differences: Different number of neutrons.
- Similarities: Same number of protons AND electrons.
🧠 Exam Technique
Read the question carefully! It asks for differences and similarities in atomic structures, not mass numbers. Examiners specifically IGNORE mentions of "different mass numbers" because mass is a property resulting from structure, not the structure itself.
❌ Common Errors
Writing "same number of neutrons" or confusing protons with nucleons. Remember: isotopes have identical chemical properties because they have the same electron configuration.
Part (c)(i) Calculating Relative Atomic Mass
Calculate the relative atomic mass of chlorine to 2 decimal places [2 marks]
✅ Correct Answer
35.48
📐 Calculation Steps
- Identify isotopic masses and abundances from the spectrum:
Isotope 35: abundance = 75.76%
Isotope 37: abundance = 24.24% - Set up the weighted mean formula:
(35 × 75.76) + (37 × 24.24) / 100 - Evaluate numerator:
(2651.6) + (896.88) = 3548.48 - Divide by 100 and format to 2 decimal places:
35.4848 → 35.48
❌ Common Errors & Traps
- Rounding too early in intermediate steps.
- Failing to follow the rubric instruction: "Give your answer to 2 decimal places". Leaving it as 35.4848 loses the final mark.
- Forgetting to divide by 100 when working with percentage data.
Part (c)(ii) Molecular Ion Peaks in Mass Spectrometry
Suggest why an M⁺ peak at m/z = 72 is observed and predict the m/z values of the other two M⁺ peaks [2 marks]
✅ Correct Answer
- Peak at m/z = 72: Contains ᶾ⁵Cl and ᶾ⁷Cl (or: ᶾ⁵Cl³⁷Cl molecular ion).
- Other two m/z values: 70 AND 74
💡 Key Knowledge
Chlorine gas exists as a diatomic molecule (Cl₂). When ionized in a mass spectrometer, it forms a molecular ion (Cl₂⁺). Since chlorine has two stable isotopes (mass 35 and mass 37), combinations give three possible molecular masses:
- 35 + 35 = 70 (ᶾ⁵Cl₂⁺)
- 35 + 37 = 72 (ᶾ⁵Clᶾ⁷Cl⁺ - can occur in 2 ways: 35-37 or 37-35)
- 37 + 37 = 74 (ᶾ⁷Cl₂⁺)
🧠 Exam Technique
Clearly state isotope combinations using mass numbers as superscripts (e.g., ᶾ⁵Clᶾ⁷Cl) to secure top-level credit for the first mark. Ensure both additional values (70 and 74) are listed clearly for the second mark.
Topics
Module 2: Foundations in chemistry · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure
Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.