Edexcel A-Level Chemistry Paper 1, November 2021: Question 1

4 marks · Medium difficulty · Calculations

Identify isoelectronic ions, select the definition of relative isotopic mass, and calculate the relative atomic mass of bromine from isotopic abundance data.

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Question

A three-part chemistry question about atoms, isotopes, and ions. Part (a) is a multiple-choice question asking to identify which pair of ions is isoelectronic among options A (N3- and Cl-), B (O2- and S2-), C (Na+ and K+), and D (Na+ and Mg2+). Part (b) is a multiple-choice question asking for the correct definition of relative isotopic mass from four statements involving carbon-12. Part (c) provides a table with isotopic mass and percentage abundance data for bromine-79 (78.918, 50.52%) and bromine-81 (80.916, 49.48%), and asks to calculate the relative atomic mass of bromine to two decimal places.
Question text

1 This is a question about atoms, isotopes and ions.

(a) Which of the following pairs of ions is isoelectronic?

(1)

A N3– and Cl–

B O2– and S2–

C Na+ and K+

D Na+ and Mg2+

(b) Which is a correct definition of relative isotopic mass?

(1)

A the weighted mean mass of an atom of an element relative to one twelfth of

the mass of an atom of the isotope carbon-12

B the mass of one atom of an isotope relative to one twelfth of the mass of an

atom of the isotope carbon-12

C the weighted mean mass of an atom of an element relative to 12g of the

isotope carbon-12

D the mass of one atom of an isotope relative to the mass of 12g of the

isotope carbon-12

(c) The percentage composition of the two bromine isotopes in a sample is given in

the table.

Isotope Relative isotopic mass Percentage abundance

bromine-79 78.918 50.52

bromine-81 80.916 49.48

Calculate the relative atomic mass of bromine in this sample.

Give your answer to two decimal places.

(2)

*P65463A0228*

(Total for Question 1 = 4 marks)

Mark scheme

Show the mark scheme The mark scheme details answers for all three parts. Part (a) indicates D is correct with explanations for why others are incorrect. Part (b) indicates B is correct with explanations for why others are incorrect. Part (c) awards 1 mark for the correct calculation expression and 1 mark for the final evaluated answer of 79.91 to 2 decimal places.

Question

Answer Mark

Number

1(a) The only correct answer is D (Na+ and Mg2+) (1)

A is not correct because the chloride ion has an extra shell of electrons compared to the nitride ion

B is not correct because the sulfide ion has an extra shell of electrons compared to the oxide ion

C is not correct because the potassium ion has an extra shell of electrons compared to the sodium ion

Question

Answer Mark

Number

1(b) The only correct answer is B (the mass of one atom of an isotope relative to one twelfth of the mass of an atom of (1)

the isotope carbon-12)

A is not correct because this is the relative atomic mass definition

C is not correct because this is part of the relative atomic mass definition and because of the comparison of one

atom to 12 g

D is not correct because of the comparison of one atom to 12 g

Question

Answer Additional Guidance Mark

Number

1(c) Example of calculation (2)

• correct expression (1) RAM= (50.52 x 78.918)+ (49.48 x 80.916)

• evaluation of calculation and answer to 2 dp (1) (= 79.9066104)

= 79.91

ALLOW units of g mol—1 only

Do not award units of %

Correct answer without working scores (2)

(Total Question 1 = 4 marks)

How to answer it

Atoms, Isotopes and Ions Study Guide

What this question tests

This assessment checks your fundamental understanding of atomic structure terminology, electronic configurations of ions (isoelectronic species), accurate definitions of isotopic mass standardisation, and your ability to calculate relative atomic mass from isotopic abundance data using correct rounding and significant figures.

Question Part (a)

Identifying Isoelectronic Ions

✅ Correct Answer

D: Na⁺ and Mg²⁺

Awarded 1 mark for selecting D.

💡 Key Knowledge

Isoelectronic means having the exact same number of electrons (and therefore the same electronic configuration).

  • Na⁺ has 11 - 1 = 10 electrons (1s² 2s² 2p⁶).
  • Mg²⁺ has 12 - 2 = 10 electrons (1s² 2s² 2p⁶).

❌ Common Errors

  • A (N³⁻ and Cl⁻): Nitride has 10 electrons, but chloride has 18 electrons. They are in different periods.
  • B & C: Comparing ions from different periods/shells where electron counts do not match.
Question Part (b)

Defining Relative Isotopic Mass

✅ Correct Answer

B: the mass of one atom of an isotope relative to one twelfth of the mass of an atom of the isotope carbon-12

Awarded 1 mark for selecting B.

🧠 Exam Technique

Watch out for the specific terminology! Definitions in physical chemistry are tightly marked. Isotopic mass refers to a single isotope (hence "mass of *one atom* of an isotope"), whereas relative atomic mass refers to a weighted mean of an element.

❌ Common Errors

Option A defines relative atomic mass (mentioning "weighted mean of an atom of an element"), not relative isotopic mass. Options C and D incorrectly reference macroscopic amounts like "12 g" instead of atomic scales.

Question Part (c)

Calculating Relative Atomic Mass

📐 Step-by-Step Calculation

  1. Identify the formula: Aᵣ = Σ (Isotopic mass × percentage abundance) / 100
  2. Substitute values:
    Aᵣ = ((50.52 × 78.918) + (49.48 × 80.916)) / 100
  3. Evaluate numerator:
    = (3986.35736) + (4003.68368) = 7990.04104
  4. Divide by 100:
    = 79.9066104
  5. Apply rounding constraint: Round to 2 decimal places to get 79.91
Mark breakdown: (1) for correct expression setup, (1) for correct evaluation and 2 dp rounding. Correct answer with no working scores full marks (2).

❌ Common Calculation Traps

  • Rounding too early: Do not round intermediate steps in your calculator. Write down the full decimal string before final rounding.
  • Unit mistakes: If you include units, they must be valid (e.g. g mol⁻¹ ). Never write percentage signs ( % ) as units for atomic mass!

Topics

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

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