Edexcel A-Level Chemistry Paper 1, June 2017: Question 1

6 marks · Medium difficulty · Synoptic Questions

Determine the sub-atomic particles in a phosphide ion, explain the shape and bond angle of phosphorus(III) chloride, identify its bond and molecular polarity, and determine the formulae and mass/charge ratios for its molecular ion peaks.

Practise this question

Question

An exam question with multiple parts about phosphorus and phosphorus(iii) chloride. Part (a) asks for the number of protons, neutrons, and electrons in a phosphide ion P3- with multiple-choice options A to D. Part (b)(i) shows a 3D structural diagram of PCl3 and asks to explain its shape and bond angle. Part (b)(ii) has a multiple-choice table for bond and molecular polarity of PCl3. Part (c) asks for the formulae and m/z values of molecular ion peaks given isotope mass numbers.
Question text

1 A phosphorus atom has mass number 31.

(a) How many of each sub-atomic particle are present in the phosphide ion, P3–?

(1)

Number of protons Number of neutrons Number of electrons

A 15 16 12

B 15 16 18

C 16 15 12

D 16 15 18

(b) Phosphorus(III) chloride molecules are pyramidal with a bond angle less than 109.5°.

P

Cl Cl

Cl

(i) Explain why a phosphorus(III) chloride molecule has this shape and bond angle.

(2)

(ii) Which describes the polarity of the P—Cl bond and the polarity of the

phosphorus(III) chloride molecule?

(1)

Polarity of P—Cl bond Polarity of molecule

A non-polar non-polar

2 B non-polar polar

C polar*P48058A0228*non-polar

D polar polar

(c) Phosphorus has one naturally occurring isotope with mass number 31.

Chlorine exists as two isotopes with mass numbers 35 and 37.

Give the formulae and mass/charge ratio of the ions responsible for the

molecular ion peaks in the mass spectrum of phosphorus(III) chloride, PCl3.

(2)

(Total for Question 1 = 6 marks)

Mark scheme

Show the mark scheme A mark scheme showing correct answers for question 1. Part (a) is option B. Part (b)(i) lists marking points for 4 pairs of electrons (3 bond pairs, 1 lone pair) minimizing repulsion and lone pair-bond pair repulsion being greater than bond pair-bond pair repulsion. Part (b)(ii) is option D. Part (c) lists the four possible molecular ion formulae and their corresponding m/z values: P(35Cl)3+ at 136, P(35Cl)2(37Cl)+ at 138, P(35Cl)(37Cl)2+ at 140, and P(37Cl)3+ at 142.

Question

Answer Mark

Number

1(a) The only correct answer is B (1)

A is not correct because incorrect electrons

C is not correct because incorrect protons, neutrons and electrons

D is not correct because incorrect protons and neutrons

Question

Answer Additional Guidance Mark

Number

1(b)(i) An explanation that makes reference to the ‘ions’ scores (0) overall (2)

following points:

(pyramidal / this shape) because there are Allow the electron pairs are arranged to minimise

4 pairs / 3 bond pairs and one lone pair of repulsion

electrons (around central / P atom) Allow (4) pairs of electrons with maximum separation /

and as far apart as possible

these are arranged to minimise repulsion

(1) Ignore reference to ‘bonds’

Ignore wrong shape

Ignore repel equally

Ignore repulsion between electrons

(bond angle less than 109.5o) as lone pair- There must be a comparison in M2

bond pair repulsion is greater than bond Allow lone pairs have greater repulsion than bond pairs

pair-bond pair repulsion (1)

Ignore just ‘the lone pairs repel more’

Ignore repetition of the question e.g. ‘reduces the

bond angle’

Ignore incorrect bond angle stated (Data book value is

100.1o)

Do not allow bond angle >109.5o

Question

Answer Mark

Number

1(b)(ii) The only correct answer is D (1)

A is not correct because both incorrect

B is not correct because non-polar bond is incorrect

C is not correct because non-polar molecule is incorrect

Question

Answer Additional Guidance Mark

Number

1(c) Example of answer: (2)

all 4 ion formulae (1) ions m/z

P(35Cl) + 136

all 4 m/z values (1) P(35Cl) 37Cl+ 138

P35Cl(37Cl) + 140

or P(37Cl) + 142

any two m/z values with corresponding ion formulae (1)

Allow any other unambiguous way

the other two m/z values with corresponding ion formulae (1) of representing the formulae e.g.

with brackets or in words

Positive charge only needs to be

shown on one of the ions

Ignore mass number on P

(Total for Question 1 = 6 marks)

How to answer it

Atomic Structure, Bonding, Shape, and Mass Spectrometry

What this question tests

This question assesses foundational A-Level Chemistry concepts across multiple topics: calculating sub-atomic particles in ions, applying VSEPR theory to deduce shapes and bond angles, determining molecular and bond polarity, and predicting molecular ion mass spectrometry (m/z) peaks based on isotopic abundances.

Question Part (a)

Sub-atomic Particles in Ions

✅ Correct Answer

Option B (Protons: 15, Neutrons: 16, Electrons: 18)

💡 Key Knowledge

  • Protons: Equal to the atomic number (Z). Phosphorus is element 15.
  • Neutrons: Mass number minus atomic number (31 - 15 = 16).
  • Electrons: For a neutral atom, electrons equal protons (15). The P³⁻ ion has gained 3 electrons, so 15 + 3 = 18.

❌ Common Errors

Students often forget to adjust the electron count for the negative charge, mistakenly selecting Option A (12 electrons) by subtracting instead of adding, or confusing mass and atomic numbers.

Mark: 1 mark
Question Part (b)(i)

Shape and Bond Angles of Phosphorus(III) Chloride (PCl₃)

✅ Correct Answer & Mark Scheme

  • Point 1: Pyramidal shape because there are 4 pairs of electrons around the central P atom (3 bond pairs and 1 lone pair) which arrange to minimise repulsion.
  • Point 2: Bond angle is less than 109.5° because lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion.

🧠 Exam Technique

To secure both marks, you must explicitly mention: (1) total electron pairs (and their arrangement), and (2) a direct comparison showing that lone pair repulsion is stronger than bond pair repulsion. Vague statements like "lone pairs repel more" without mentioning what they repel against can lose credit.

❌ Common Errors

Writing that "ions" cause the repulsion (PCl₃ is a covalent molecule, not ionic!). Another frequent error is failing to make an explicit comparison between the types of repulsion.

Marks: 2 marks
Question Part (b)(ii)

Polarity of Bonds and Molecules

✅ Correct Answer

Option D (P—Cl bond is polar; molecule is polar)

💡 Key Knowledge

Phosphorus and chlorine have different electronegativities, making the P—Cl bond polar. Because the molecule is symmetrical in terms of vector cancellation (it is pyramidal, not tetrahedral or trigonal planar), the bond dipoles do not cancel out, resulting in a net dipole moment for the molecule.

Mark: 1 mark
Question Part (c)

Mass Spectrometry of PCl₃ Molecular Ions

📐 Step-by-Step Calculation of m/z Peaks

  1. Identify isotopes: Phosphorus has one isotope ( ³¹P ). Chlorine has two isotopes ( ³⁵Cl and ³⁷Cl ).
  2. Calculate combinations for PCl₃⁺:
    • P(³⁵Cl)₃ = 31 + (3 × 35) = 136
    • P(³⁵Cl)₂(³⁷Cl) = 31 + (2 × 35) + 37 = 138
    • P(³⁵Cl)(³⁷Cl)₂ = 31 + 35 + (2 × 37) = 140
    • P(³⁷Cl)₃ = 31 + (3 × 37) = 142

🧠 Exam Technique & Formatting

Make sure to include the positive charge on your ion formulae (e.g., P(³⁵Cl)₃⁺ or written clearly in words/brackets). One mark is awarded for all correct formulae, and one mark for all corresponding m/z values. (Partial credit: 2 correct pairs get 1 mark).

❌ Common Calculation Traps

Forgetting to show the charge on the ion, or incorrectly calculating the isotopic mass combinations by missing out permutations of chlorine-35 and chlorine-37.

Marks: 2 marks

Topics

Physical Chemistry · Organic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure · Topic 7: Modern Analytical Techniques I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.