Edexcel A-Level Chemistry AS Paper 1, June 2016: Question 2

10 marks · Medium difficulty · Calculations

Identify errors in an old chemistry excerpt, determine the structure of a carbon-13 ion, define relative atomic mass, calculate relative atomic mass from isotopic sample data, and explain a peak in a bromine mass spectrum.

Practise this question

Question

Exam question 2 about atomic structure and isotopes, containing parts (a) to (d). Part (a) asks to identify and correct two errors in a 1938 book excerpt about electrons and isotopes. Part (b) is a multiple-choice question on the structure of a 1+ carbon-13 ion. Part (c)(i) defines relative atomic mass, and (c)(ii) provides a 5.000 g lithium sample data table to calculate the relative atomic mass. Part (d) asks to explain a very small peak at m/z = 80 in a bromine mass spectrometer analysis.
Question text

2 This question is about the structure of the atom and isotopes.

The following excerpt is taken from the book Inorganic Chemistry by Bailey and

Snellgrove, fourth impression 1938.

“Some of the electrons are also contained in the nucleus, whilst the

remainder are … arranged in rings revolving round the nucleus … The two

isotopes [of chlorine] have therefore 18 and 20 electrons respectively in the

nucleus and 17 [electrons] external to it.”

(a) Identify and correct two errors in the excerpt.

(2)

(b) What is the structure of a 1+ ion of the carbon-13 isotope?

(1)

A six protons, six neutrons and five electrons

B six protons, seven neutrons and six electrons

C six protons, seven neutrons and five electrons

D seven protons, six neutrons and six electrons

(c) (i) State what is meant by the term relative atomic mass.

(2)

(ii) A 5.000 g sample of lithium, containing the two isotopes lithium-6 and

lithium-7, was found to contain 0.460 g of the isotope lithium-6.

Calculate the relative atomic mass of lithium for this sample. Give your answer

to an appropriate number of significant figures.

4 Isotope Relative isotopic mass

*P49837A0420*Lithium-66.015

Lithium-7 7.016

(3)

(d) A mass spectrometer was used to analyse a sample of bromine, Br2, with only the

79Br and 81Br isotopes present.

Explain why a very small peak occurs at m/z = 80.

(2)

(Total for Question 2 = 10 marks)

Mark scheme

Show the mark scheme Mark scheme for question 2 showing acceptable answers, points, and calculations for parts (a) through (d), including specific acceptable statements for correcting historical excerpts, the correct multiple-choice option C, the definition of relative atomic mass, step-by-step calculation of relative atomic mass for lithium, and explanation of the Br2^2+ ion causing the peak at m/z = 80.

How to answer it

Structure of the Atom and Isotopes Study Guide

What this question tests

This question assesses your fundamental understanding of atomic structure, subatomic particles (protons, neutrons, and electrons), the definition of relative atomic mass, isotopic mass calculations, and the analysis of mass spectrometry data including doubly-charged ions.

Question Part (a)

Identifying Historical Errors in Atomic Models

✅ Correct Answers (Any Two)

  • No electrons are found within the nucleus.
  • Chlorine isotopes have 18 and 20 neutrons (not electrons).
  • Electrons are not in rings around the nucleus, but in orbitals / sub-shells / energy levels.

❌ Common Errors

Students often lose marks by providing vague corrections such as "electrons are outside" without specifying where they actually reside (orbitals/shells) or incorrectly addressing the neutron count in chlorine isotopes.

Mark: 2 marks (1 mark for each valid correction identified, max 2)
Question Part (b)

Structure of a 1+ Ion of Carbon-13

✅ Correct Answer

C — six protons, seven neutrons and five electrons.

💡 Key Knowledge

  • Protons: Atomic number of carbon is 6, so it always has 6 protons.
  • Neutrons: Carbon-13 has mass number 13, so 13 - 6 = 7 neutrons.
  • Electrons: A neutral carbon atom has 6 electrons. A 1+ ion has lost one electron, leaving 5 electrons.
Mark: 1 mark
Question Part (c)(i)

Definition of Relative Atomic Mass

✅ Correct Answers

  • The weighted average / mean mass of an atom of an element.
  • Compared to 1/12th the mass of an atom of carbon-12.

🧠 Exam Technique & Pitfalls

You must use the word atom at least once to score both marks. Examiners strictly reject answers referring to "mass of an element" instead of an "atom of an element". Never write "12g" for carbon-12; it must be relative to 1/12th the mass of a carbon-12 atom.

Mark: 2 marks
Question Part (c)(ii)

Calculating Relative Atomic Mass from Sample Masses

📐 Step-by-Step Calculation

  1. Find the mass of Lithium-7: 5.000 g - 0.460 g = 4.540 g
  2. Calculate percentages (or use raw masses in formula):
    Li-6 % = (0.460 / 5.000) × 100 = 9.2%
    Li-7 % = (4.540 / 5.000) × 100 = 90.8%
  3. Apply the RAM formula:
    RAM = ((0.092 × 6.015) + (0.908 × 7.016)) = 6.9239...
    *(Alternatively: ((0.460 × 6.015) + (4.540 × 7.016)) / 5.000 )*
  4. Round appropriately:
    Give to 2, 3, or 4 significant figures: 6.92 or 6.9 .

❌ Common Calculation Traps

  • Forgetting to calculate the mass or percentage of the second isotope (Li-7).
  • Failing to give the final answer to an appropriate number of significant figures (2 to 4 s.f. required here).
  • Including incorrect units like g mol⁻¹ in the final answer (RAM is a ratio and has no units).
Mark: 3 marks (1 for percentages/numerator, 1 for correct method/RAM, 1 for correct evaluation to 2/3/4 s.f.)
Question Part (d)

Mass Spectrometry and Doubly-Charged Ions

✅ Correct Answers

  • Br₂ has a molecular mass of 160 (from one ⁷⁹Br and one ⁸¹Br atom, or two ⁸⁰Br if considered, though isotopes are 79 and 81).
  • A very small peak at m/z = 80 is caused by the doubly-charged ion Br₂²⁺ (since 160 / 2 = 80).

💡 Key Knowledge

Mass spectrometers can occasionally knock off more than one electron during ionization. When a diatomic molecule like Br₂ (mass 160) loses 2 electrons to form a 2+ ion, its mass-to-charge ratio ( m/z ) is halved: 160 / 2 = 80 .

Mark: 2 marks (1 mark for identifying the mass 160 molecule, 1 mark for linking m/z = 80 to the doubly-charged ion Br₂²⁺ )

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2016. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.