Edexcel A-Level Chemistry Paper 1, June 2018: Question 2

8 marks · Medium difficulty · Short Open Response

Deduce subatomic particle numbers for lithium isotopes, identify element X and formulas of species in a mass spectrum of X2, and state maximum electron capacities for specific atomic regions.

Practise this question

Question

A three-part chemistry exam question about atoms, molecules, and ions. Part (a) asks to complete a table for the number of protons, neutrons, and electrons in 6Li atom and 7Li+ ion. Part (b) gives m/z values 32, 33, 34, 35, 36 for a diatomic molecule X2+ and asks to deduce the formulae of all species and identify element X. Part (c) asks to complete a table showing the maximum number of electrons in the 1s orbital, 2p subshell, and the third quantum shell.
Question text

2 This question is about atoms, molecules and ions.

(a) Lithium exists as two isotopes.

Complete the table to show the numbers of subatomic particles in a 6Li atom and

a 7Li+ ion.

(2)

Particle Protons Neutrons Electrons

6Li

7Li+

(b) The mass spectrum of a diatomic molecule, X2, has peaks at the following

m / z values for the X + ion:

32, 33, 34, 35, 36

Deduce the formulae of all the species responsible for each of the peaks in the

mass spectrum of X2, identifying element X and showing clearly the isotopes present.

(3)

(c) Complete the table to show the maximum number of electrons which can fill

each region of an atom.

(3)

Region Maximum number of electrons

the 1s orbital

the 2p subshell

the third quantum shell

(Total for Question 2 = 8 marks)

Mark scheme

Show the mark scheme The mark scheme providing correct answers for question 2 parts a, b, and c. Part a lists protons, neutrons, and electrons for 6Li and 7Li+. Part b identifies oxygen as element X and provides isotopic combinations for each m/z value. Part c gives the electron capacities as 2 for 1s orbital, 6 for 2p subshell, and 18 for the third quantum shell.

Question

Acceptable Answer Additional Guidance Mark

Number

2(a) Example of table (2)

6Li – 3 protons and 3 neutrons and 3 electrons

(1)

7Li+ – 3 protons and 4 neutrons and 2 electrons (1)

If no oher mark is scored, allow (1) for any 4

correct numbers

Ignore + or - signs

Question

Acceptable Answer Additional Guidance Mark

Number

2(b) An answer that makes reference to the following points: Isotopes in ions at each m/z value: (3)

(32 –) 16O=16O+ / 16O +

identification of oxygen / O (1) (33 –) 16O=17O+

(34 –) 16O=18O+ and 17O=17O+ / 17O +

2 +

identification of isotopes corresponding to any 3 m/z (35 –) 17O=18O+

values (1) (36 –) 18O=18O+ / 18O +

Conditional on M2 awarded Allow single bonds

identification of isotopes corresponding to other 2 m/z

values (1) Allow any other unambiguous ways of

showing the masses of the isotopes for each

m/z value e.g. 16+16, O 16

Allow use of X / another symbol e.g. Cl

instead of O for M2 and M3

Ignore missing charges as given in question

Penalise negative charge once only

Question

Acceptable Answer Additional Guidance Mark

Number

2(c) Example of table (3)

1s orbital – 2 electrons (1)

2p subshell – 6 electrons (1)

third quantum shell – 18 electrons (1)

Allow 1s2 for 2

Allow 2p6 for 6

Ignore 3s23p63d10 for the third number

Do not award more than one number

written in the box e.g. 8 or 18 in the third

(Total for Question 2 = 8 marks)

How to answer it

Atoms, Molecules and Ions Study Guide

📌 What this question tests

This question assesses fundamental atomic structure and mass spectrometry knowledge at A-Level. You are tested on subatomic particle accounting in neutral atoms vs. ions, interpreting mass spectra of diatomic molecules to deduce isotopic compositions, and recalling maximum electron capacities across orbitals, subshells, and principal quantum shells.

Question Part (a)

Subatomic Particles in Atoms and Ions

Determine numbers of protons, neutrons, and electrons for ⁶Li and ⁷Li⁺.

✅ Correct Answer

  • ⁶Li atom: 3 protons, 3 neutrons, 3 electrons
  • ⁷Li⁺ ion: 3 protons, 4 neutrons, 2 electrons

💡 Key Knowledge

  • Atomic number (bottom number/periodic table number) = protons = 3 for Lithium.
  • Mass number minus atomic number = neutrons ( 6 - 3 = 3 ; 7 - 3 = 4 ).
  • Positive ions have fewer electrons than protons due to loss of outer electron(s) ( 3 - 1 = 2 ).

❌ Common Errors

  • Confusing the mass number for the number of neutrons.
  • Failing to adjust the electron count for the positive charge ( ⁷Li⁺ has 2 electrons, not 3).
Mark Allocation (2 marks): (1) mark for any 4 correct entries in the table. Full (2) marks for all 6 entries correct. Examiner note: ignore + or - signs when checking particle numbers.
Question Part (b)

Mass Spectrometry of Diatomic Molecules

Deduce formulae for species responsible for peaks at m/z values 32, 33, 34, 35, and 36 for X₂⁺.

✅ Correct Answer

  • m/z 32: ⁴⁶O=¹⁶O⁺ (or ¹⁶O₂⁺)
  • m/z 33: ¹⁶O=¹⁷O⁺
  • m/z 34: ¹⁶O=¹⁸O⁺ AND ¹⁷O=¹⁷O⁺
  • m/z 35: ¹⁷O=¹⁸O⁺
  • m/z 36: ¹⁸O=¹⁸O⁺

🧠 Exam Technique & Strategy

  1. Identify the element: Dividing the lowest peak (32) by 2 gives 16, which corresponds to Oxygen (O).
  2. List common oxygen isotopes: ¹⁶O, ¹⁷O, and ¹⁸O.
  3. Combine isotope pairs: Systematically sum combinations to match each m/z value:
    • 32 = 16 + 16
    • 33 = 16 + 17
    • 34 = 16 + 18 (or 17 + 17)
    • 35 = 17 + 18
    • 36 = 18 + 18

❌ Common Errors

  • Omitting one of the two valid combinations for m/z 34 ( ¹⁶O=¹⁸O and ¹⁷O=¹⁷O ).
  • Forgetting to include the positive charge symbol ( ⁺ ) on molecular ions when writing formulae.
Mark Allocation (3 marks): (1) Mark for identifying Oxygen / O. (1) Mark for identifying isotopes for any 3 m/z values. (1) Conditional on M2, identifying isotopes for the remaining m/z values. Note: Allow single bonds, unspaced representations, or X as a placeholder if oxygen wasn't named, provided combinations are mathematically consistent.
Question Part (c)

Electron Capacities in Quantum Regions

Complete the table showing maximum electron capacities for specific atomic regions.

✅ Correct Answer

  • 1s orbital: 2 electrons
  • 2p subshell: 6 electrons
  • Third quantum shell: 18 electrons

💡 Key Knowledge

  • Orbitals: Hold a maximum of 2 electrons (with opposite spins).
  • Subshells (s, p, d, f): Contain 1, 3, 5, and 7 orbitals respectively, giving maximum capacities of 2, 6, 10, and 14 electrons.
  • Quantum Shells: Follow the formula 2n² . For the 3rd shell ( n = 3 ), capacity = 2(3)² = 18 electrons.

❌ Common Errors

  • Confusing subshell capacity with principal shell capacity (e.g., thinking the 3rd shell only holds 8 electrons due to the octet rule in GCSE chemistry).
  • Mixing up subshell names and orbital counts.
Mark Allocation (3 marks): 1 mark per correct row entry. Accept configuration notation such as 1s² or 2p⁶ as long as the electron number is clear. Ignore electronic configurations like 3s²3p⁶3d¹⁰ if listed as extra text.

Topics

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

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.