AQA GCSE Chemistry Chemistry Paper 1 (Foundation), November 2021: Question 9
9 marks · Standard Demand difficulty · Short Answer
Define isotopes, calculate the relative atomic mass of gallium from isotopic abundances, deduce electron and neutron numbers, identify the gallium ion formula, and explain why gallium's discovery supported Mendeleev's periodic table.
Practise this questionQuestion
Question text
09 This question is about atomic structure and the periodic table.
Gallium (Ga) is an element that has two isotopes.
09.1 Give the meaning of ‘isotopes’.
You should answer in terms of subatomic particles.
[2 marks]
09.2 Table 8 shows the mass numbers and percentage abundances of the
isotopes of gallium.
Table 8
Mass number Percentage abundance (%)
69 60
71 40
Calculate the relative atomic mass (Ar) of gallium.
Give your answer to 1 decimal place.
[2 marks]
Relative atomic mass (1 decimal place) =
Gallium (Ga) is in Group 3 of the modern periodic table.
09.3 69
Give the numbers of electrons and neutrons in an atom of the isotope 31Ga
[2 marks]
*36* Number of electrons
Number of neutrons
09.4 What is the most likely formula of a gallium ion?
[1 mark]
Tick ( ) one box.
Ga+
Ga–
Ga3+
Ga3–
09.5 Gallium was discovered six years after Mendeleev published his periodic table.
Give two reasons why the discovery of gallium helped Mendeleev’s periodic table to
become accepted.
[2 marks]
Mark scheme
Show the mark scheme
Question 9
AO /
Question Answers Extra information Mark
Spec. Ref.
09.1 (atoms with the) same number allow atoms with the same 1 AO1
of protons atomic number 4.1.1.4
allow atoms of the same 4.1.1.5
element
ignore the same number of
electrons
(but with) different numbers of ignore (but with) different mass 1
neutrons numbers
do not accept (but with) different
relative atomic mass
AO /
Spec. Ref.
09.2 (69 × 60) + (71 × 40) 1 AO2
(Ar =)
100 4.1.1.6
= 69.8 1
AO /
Spec. Ref.
09.3 (number of electrons) = 31 1 AO2
4.1.1.4
(number of neutrons) = 38 1 4.1.1.5
AO /
Spec. Ref.
09.4 Ga3+ 1 AO3
4.2.1.2
Question 9 continued
AO /
Spec. Ref.
24 09.5 (gallium) fitted in a gap 1 AO2
(Mendeleev had left) 4.1.2.2
(gallium’s) properties were allow (gallium’s) properties 1
predicted correctly (by matched the rest of the group
Mendeleev)
Total 9
How to answer it
Atomic Structure, Isotopes & The Periodic Table
What this question tests
This question assesses fundamental recall and mathematical application across Paper 1 Chemistry:
- Defining isotopes precisely in terms of subatomic particles.
- Calculating relative atomic mass (Aᵣ) from percentage abundances and rounding appropriately.
- Deducing electron and neutron counts from standard isotopic nuclear symbol notation.
- Predicting ion formation and charges using group number and electron configurations.
- Explaining the historical significance of Mendeleev's predictions and how newly discovered elements validated his periodic table.
Defining Isotopes
Meaning of 'isotopes' in terms of subatomic particles
✅ Correct Answer (Mark Scheme)
- (Atoms with the) same number of protons [1 mark]
- (but) different numbers of neutrons [1 mark]
Do NOT accept: "different relative atomic mass".
❌ Common Errors & Traps
- Ignoring the prompt instruction: The question strictly states "in terms of subatomic particles". Writing "atoms with different mass numbers" scores 0 marks.
- Mentioning electrons only: While neutral isotopes have equal numbers of electrons, the definition strictly hinges on protons.
Calculating Relative Atomic Mass (Aᵣ)
Calculating Aᵣ from isotopic abundance to 1 decimal place
📐 Step-by-Step Calculation
Data: ⁶⁹Ga = 60%, ⁷¹Ga = 40%
- Set up the abundance formula:
Aᵣ = Σ (mass × abundance) / 100
Aᵣ = [(69 × 60) + (71 × 40)] / 100 [1 mark] - Calculate total mass:
(4140 + 2840) / 100 = 6980 / 100 - Final value & rounding:
69.8 [1 mark]
🧠 Exam Technique & Sanity Check
- Range Check: The answer must lie strictly between the mass numbers 69 and 71. Since 69 is more abundant (60%), the value must be closer to 69 than to 71. 69.8 makes logical sense.
- Decimal Places: The question asks for 1 decimal place. Here, 69.8 is already given to 1 d.p. Never round 69.8 up to 70!
Subatomic Particles in an Atom
Deducing numbers of electrons and neutrons from ⁶⁹₃₁Ga
✅ Correct Answers
- Number of electrons: 31 [1 mark]
- Number of neutrons: 38 [1 mark]
💡 Key Knowledge
For any isotope notation ᴬ_Z X:
- Bottom number (Z, Atomic Number) = number of protons = 31. In an uncharged neutral atom, number of electrons = number of protons = 31.
- Top number (A, Mass Number) = protons + neutrons = 69.
- Number of neutrons = Mass number − Atomic number = 69 − 31 = 38.
Predicting Ion Formulae
Identifying the most likely ion formed by a Group 3 element
✅ Correct Choice
Tick: Ga³⁺ [1 mark]
🧠 Logic & Electron Rules
- Gallium is in Group 3, meaning it has 3 electrons in its outer shell.
- Metals lose electrons to form positive ions (cations) with a full outer shell.
- Losing 3 negatively charged electrons leaves a 3+ charge (Ga → Ga³⁺ + 3e⁻).
- Trap: Ga³⁻ represents gaining 3 electrons (which non-metals in Group 5 do, not metals in Group 3).
Mendeleev's Periodic Table
Why the discovery of gallium helped Mendeleev's table become accepted
✅ Correct Answer (Any 2 Points)
- (Gallium) fitted into a gap that Mendeleev had left. [1 mark]
- (Gallium's) properties were predicted accurately by Mendeleev (or matched the rest of the group). [1 mark]
💡 Examiner Insight
Mendeleev famously predicted the existence and properties of an unknown element he called "eka-aluminium".
When gallium was discovered in 1875, its density, melting point, and chemical reactivity matched his predictions almost identically. This provided powerful empirical evidence that convinced the scientific community his periodic arrangement was correct.
Topics
Chemistry · C1: Atomic Structure and the Periodic Table · C2: Bonding, Structure and the Properties of Matter
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Foundation), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.