AQA AS Level Physics Paper 1, June 2022: Question 2
6 marks · Medium difficulty · Short Answer
Explain ionisation of bromine gas by an electron beam, calculate the number of neutrons in its heavier isotope, and deduce the percentage of each isotope from molecular abundance data.
Practise this questionQuestion
Question text
02 A sample of bromine gas contains a mixture of two isotopes. An experiment is done
to find the percentage of each isotope in this sample.
02.1 In the experiment, the gas is ionised by a beam of electrons.
Explain how the beam of electrons causes a particle of the gas to have a charge
of +1e.
[2 marks]
The gas consists of bromine molecules. Each molecule has two bromine atoms.
The experiment finds that the bromine molecules contain 158, 160 or 162 nucleons.
Figure 1 shows the percentage of these different molecules in the sample.
Figure 1
02.2 Bromine has a proton number of 35
The two isotopes in the sample have different nucleon numbers.
Calculate the number of neutrons for the isotope that has the greater nucleon number.
[2 marks]
number of neutrons =
02.3 Deduce the percentage of each isotope in the gas.
Justify your conclusion.
[2 marks]
Mark scheme
Show the mark scheme
Question Answers Additional Comments/Guidance Mark AO
02.1 An electron in the beam collides with an electron in the gas MP1 is awarded for the description of the 2 1 x AO1
particle. electron-electron interaction or the resulting 1 x AO2
energy transfer between these electrons.
OR
Treat the gas particles are ‘excited’ as neutral,
An electron in the beam transfers (some of its kinetic) energy
must mention an interaction between beam
to an electron in the gas particle
electron and (atomic) electron or an energy
transfer from beam electron to (atomic)
electron as cause of excitation
Allow beam electron collides with / transfers
energy to gas (particle) causing an atomic
electron to gain energy
Condone use of plurals in MP1
Penalise more than one electron leaving a gas
particle
One (atomic) electron leaves the gas particle Condone
One (atomic) electron leaves the gas (atom)/
the gas (particle) has lost one electron
Physics errors that relate the effect to
annihilation or beta decay or PEE or electron
capture gain zero marks.
Alternative for MP1:
02.2 2 2 x AO2
Finds the nucleon number of the more massive isotope: subtracts proton number from their nucleon
number / subtracts total number of protons
162 ÷ 2 = 81
from total number of nucleons.
eg 80 − 35 or 79 – 35 or 160 – 70 or 158− 70
Condone 45 or 44 on answer line without
working for one mark.
Do not allow 162 − 35 or 160 − 35 or
158 − 35
OR
Condone 92 on answer line without
162 − (2 × 35) = 92
working for 1 mark.
90 or 88 on answer line without working no
marks
(answer =) 46 c.a.o
Do not allow 50% of 158 and 50% of 162
02.3 The percentage is the same for both isotopes / each isotope 2 2 x AO3
makes up 50% of the gas (by number) Where percentage stated must be 50 %
Do not allow more than 2 isotopes
158 is made of two atoms of the lighter isotope and 162 is
made of two atoms of the heavier isotope and the percentages
of 158 and 162 are: both 25% / both same /present in the same
ratio
Or words to that effect
OR
Half of the 160 is made from the lighter isotope and all of the
158 is made from the lighter isotope (totalling 50%)
OR Accept equivalent discussion in terms of
numbers of neutrons present in nuclei in
Half of the 160 is made from the heavier isotope and all of the
molecules / nucleon numbers of nuclei in
162 is made from the heavier isotope (totalling 50%)
molecules.
Restating the percentages of the molecules is
insufficient for MP2.
Total 6
How to answer it
Study Guide: Particles & Isotopes Analysis
This question assesses your understanding of ionisation processes in particle physics, atomic structure (protons, neutrons, and nucleons), and data analysis involving isotopic abundances in molecular gases. You must combine knowledge of particle interactions with mathematical deduction.
Ionisation by Electron Bombardment
Explain how the beam of electrons causes a particle of the gas to have a charge of +1e.
✅ Correct Answer
- An electron in the beam collides with/transfers kinetic energy to an electron in the gas particle.
- One atomic electron leaves the gas particle.
💡 Key Knowledge
Ionisation requires energy transfer to eject an orbital electron. In standard mass spectrometry setups, high-speed beam electrons knock out outer shell electrons, leaving a positively charged ion behind ( +1e ).
🧠 Exam Technique
This is a 2-mark point-scoring process. Mark 1 is awarded for describing the collision/energy transfer. Mark 2 is awarded for stating that an electron leaves the particle. Keep your language precise—avoid vague terms like "atom gets excited" without mentioning the electron leaving.
❌ Common Errors
Students often lose marks by stating the whole atom/molecule absorbs the electron, or confusing ionisation with beta decay, annihilation, or electron capture.
Calculating Number of Neutrons
Calculate the number of neutrons for the isotope that has the greater nucleon number.
✅ Correct Answer
Answer: 46
📐 Step-by-Step Calculation
- Identify the heavier molecule's nucleon number: From Figure 1, the maximum molecular nucleon number is 162 .
- Find the nucleon number of a single atom: Since bromine molecules are diatomic (Br₂), divide by 2: 162 ÷ 2 = 81 .
- Subtract the proton number: We are given the proton number is 35 .
Neutrons = 81 - 35 = 46 .
🧠 Exam Technique
Always double-check whether the value given in a graph represents a molecule or a single atom/isotope. Because bromine exists as a diatomic molecule, failing to divide by 2 is the most common trap.
❌ Common Errors
Subtracting the proton number 35 directly from the molecular nucleon number ( 162 - 35 = 92 ) without accounting for the diatomic nature of the molecule.
Deducing Isotope Percentages
Deduce the percentage of each isotope in the gas. Justify your conclusion.
✅ Correct Answer
Answer: 50% for each isotope (or both isotopes make up 50% of the gas by number).
💡 Key Knowledge
Bromine naturally consists of two isotopes. When they form diatomic molecules (Br₂), probability dictates the distribution shown in Figure 1: lighter-lighter (158), mixed (160), and heavier-heavier (162) in a 25%:50%:25% ratio (resembling a binomial expansion).
🧠 Exam Technique
To secure the second mark for justification, you must explain how the molecular abundances (25% for 158, 50% for 160, 25% for 162) break down into atomic contributions (e.g., half of the 160 peak comes from each isotope, combined with 100% of the respective pure molecules).
❌ Common Errors
Simply restating the numbers from the graph without linking them to individual atomic isotope percentages will deny you the justification mark.
Topics
Physics · 3.2 Particles and radiation
Question and mark scheme from the AQA AS Level Physics examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.