Edexcel A-Level Chemistry AS Paper 1, June 2017: Question 5
9 marks · Medium difficulty · Calculations
Complete the electronic structure of magnesium, identify its metallic bonding, define relative isotopic mass, determine subatomic particles for a specific isotope, and calculate an unknown relative isotopic mass from abundance data.
Practise this questionQuestion
Question text
5 This question is about magnesium.
(a) (i) Complete the electronic structure of a magnesium atom.
(1)
1s2
(ii) The bonding in magnesium results from
(1)
A strong electrostatic attractions between oppositely charged ions
B strong electrostatic attractions between the nuclei of magnesium atoms
and a shared pair of electrons
C strong electrostatic attractions between positively charged ions and a sea
of delocalised electrons
D weak dispersion forces between magnesium atoms
(b) Magnesium exists as three stable isotopes. One isotope has a relative
isotopic mass of 25.0.
State what is meant by the term relative isotopic mass.
(2)
(c) What are the numbers of protons, neutrons and electrons in an atom of
magnesium which has a mass number of 25?
(1)
A 12 protons, 13 neutrons and 12 electrons
B 12 protons, 25 neutrons and 12 electrons
C 13 protons, 12 neutrons and 13 electrons
D 25 protons, 13 neutrons and 12 electrons.
(d) The relative atomic mass of a sample of magnesium was found to be 24.3.
The percentage composition for two of the three isotopes is given in the table.
12 Use these data to calculate the percentage composition of the third isotope and
hence its relative isotopic mass. Give your answer to an appropriate number of*P49858A01224*
significant figures. You must show your working.
Relative isotopic mass Percentage abundance
25.0 10.00
26.0 11.01
(4)
(Total for Question 5 = 9 marks)
Mark scheme
Show the mark scheme
Question
Acceptable Answer Additional Guidance Mark
Number
5 (a) (i) (1s2) 2s2 2p6 3s2 ALLOW 1s2 repeated (1)
Question
Answer Mark
Number
5 (a) (ii) 5(a)(ii). The only correct answer is C (1)
A is not correct because this describes ionic bonding and magnesium has metallic bonding
B is not correct because this describes covalent bonding and magnesium has metallic bonding
D is not correct because this describes intermolecular forces and magnesium has metallic bonding
Question
Acceptable Answer Additional Guidance Mark
Number
5 (b) An answer that makes reference to the following points: (2)
Mass of an atom of an isotope (1) Atom needs only to be mentioned in
MP1.
Reject just "average / mean mass of
an atom"
But allow
"average / mean mass of an atom of
an isotope"
relative to 1/12th mass of an atom of carbon-12. (1) Ignore mention of moles throughout
and 12g in respect to carbon-12.
OR
Mass of one atom of an isotope x 12 (2)
Mass of one atom of carbon-12
Question
Answer Mark
Number
5 (c) 5(c). The only correct answer is A (1)
B is not correct because this has 25 neutrons and not 13
C is not correct because this has 13 protons and not 12
D is not correct because this has 25 protons and not 12
Question
Acceptable Answer Additional Guidance Mark
Number
5 (d) Example of calculation (4)
calculates percentage of 3rd (100 - (10.00 + 11.01)) = 78.99
isotope (1)
this is a standalone mark
lays out suitable equation (78.99 x isotopic mass) + (25.0 x 10.00) + (26.0 x 11.01) = 24.3
including unknown 1) 100
consolidates (1) 78.99 x isotopic mass = 1893.74
24.0 (1) Isotopic mass = 1893.74 = 24.0 (23.97443)
78.99 must be 3 s.f
Correct answer with some further working scores last 3 marks
Correct answer with no working scores last mark
Allow 24 provided there is clear calculation.
(Total for Question 5 = 9 marks
How to answer it
Edexcel AS Level Chemistry Study Guide: Properties of Magnesium & Isotopes
What this question tests
This question evaluates core foundational physical chemistry concepts: electron configurations using subshells, types of chemical bonding in metals, definitions of relative isotopic mass, subatomic particle accounting, and multi-step relative atomic mass (Ar) calculations involving percentage abundances and significant figures.
Part (a)(i): Electronic Structure
Complete the electronic structure of a magnesium atom.
✅ Correct Answer
1s² 2s² 2p⁶ 3s²
💡 Key Knowledge
Magnesium has an atomic number of 12 (12 electrons total). Filling orbitals in order of increasing energy (Aufbau principle) gives two electrons in 1s, two in 2s, six in 2p, and two in 3s.
Part (a)(ii): Bonding in Magnesium
The bonding in magnesium results from...
✅ Correct Answer
C - strong electrostatic attractions between positively charged ions and a sea of delocalised electrons
❌ Common Errors & Distractor Analysis
- A is incorrect: Describes ionic bonding (oppositely charged ions). Magnesium is a metal, not an ionic lattice.
- B is incorrect: Describes covalent bonding (shared pair of electrons).
- D is incorrect: Describes weak intermolecular dispersion forces (noble gases / simple molecular lattices).
Part (b): Defining Relative Isotopic Mass
State what is meant by the term relative isotopic mass.
✅ Correct Answer
- The mass of an atom of an isotope (1 mark)
- Relative to 1/12th the mass of an atom of carbon-12 (1 mark)
- Alternatively: (Mass of one atom of an isotope × 12) / (Mass of one atom of carbon-12) scores both marks.
🧠 Exam Technique & Strict Phrasing
Examiners are very strict here! You must include the word atom when referring to the isotope and carbon-12. Reject definitions that refer broadly to "average mass of an atom" or mention "moles" / "grams".
Part (c): Subatomic Particles
What are the numbers of protons, neutrons and electrons in an atom of magnesium which has a mass number of 25?
✅ Correct Answer
A - 12 protons, 13 neutrons and 12 electrons
💡 Key Knowledge
- Protons = Atomic number = 12 for magnesium (constant).
- Electrons = Protons in a neutral atom = 12.
- Neutrons = Mass Number − Protons = 25 − 12 = 13.
Part (d): Relative Atomic Mass Calculation
Calculate the percentage composition of the third isotope and hence its relative isotopic mass.
✅ Correct Answer
Final Isotopic Mass = 24.0 (must be given to 3 significant figures)
📐 Step-by-Step Calculation
- Find percentage of the 3rd isotope:
100 − (10.00 + 11.01) = 78.99% (1 mark - standalone) - Set up the Ar equation with an unknown (x):
(78.99 × x) + (25.0 × 10.00) + (26.0 × 11.01) / 100 = 24.3 (1 mark) - Consolidate the numerator terms:
78.99 × x = 2430 − 250 − 286.26 = 1893.74 (1 mark) - Solve and round to appropriate sig figs:
x = 1893.74 / 78.99 = 23.97443... = 24.0 (1 mark for correct 3 s.f. value)
❌ Common Calculation Traps & Marks Lost
- Significant Figures Trap: The question explicitly requests an "appropriate number of significant figures". Since input data (like 24.3 and 25.0) are given to 3 s.f., leaving your final answer as 23.97 or 24 will lose you the final accuracy mark!
- Working out marks: Even if minor arithmetic slips happen earlier, clear layout allows examiners to award consequential working marks. Always show your formula substitution clearly.
Topics
Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.