OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 21

8 marks · Medium difficulty · Structured Questions

Calculate the relative atomic mass of silicon, determine atomic structure, draw a dot-and-cross diagram, predict the shape of phosgene, and explain why elements belong to the p-block.

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Question

A structured chemistry question about p-block elements divided into parts (a), (b), and (c). Part (a)(i) gives a table with isotope 30-Si requiring protons, neutrons, and electrons. Part (a)(ii) provides percentage abundances for 28-Si, 29-Si, and 30-Si to calculate relative atomic mass. Part (b)(i) shows the displayed formula of phosgene (COCl2) and asks for a dot-and-cross diagram. Part (b)(ii) asks for the shape and explanation of the phosgene molecule. Part (c) asks why silicon, carbon, oxygen, and chlorine are classified as p-block elements.
Question text

21 This question is about elements from the p-block of the periodic table.

(a) Silicon exists as a mixture of three isotopes, 28Si, 29Si and 30Si.

(i) Complete the table to show the atomic structure of 30Si.

Protons Neutrons Electrons

30Si

[1]

(ii) A sample of silicon is analysed by mass spectrometry.

The mass spectrum shows peaks with the relative abundances below.

• 28Si 92.23%

• 29Si 4.68%

• 30Si 3.09%

Calculate the relative atomic mass of silicon in the sample.

Give your answer to two decimal places.

relative atomic mass = … [2]

(b) Phosgene, COCl2, exists as simple molecules.

The displayed formula of a phosgene molecule is shown below.

Cl

C O

Cl

(i) Draw a ‘dot-and-cross’ diagram of a phosgene molecule.

Show outer electrons only.

[1]

(ii) Name the shape of a phosgene molecule and explain why it has this shape.

Name of shape …

Explanation …

… [3]

(c) Why are silicon, carbon, oxygen and chlorine all classified as p-block elements?

… [1]

Mark scheme

Show the mark scheme The mark scheme provides answers for question 21 parts a to c. It lists the correct values for 30-Si atomic structure, the calculation steps for the relative atomic mass of silicon giving 28.11, the expected dot-and-cross diagram for phosgene including lone pairs, trigonal planar shape with electron pair repulsion explanation, and the definition of p-block elements having their highest energy electron in a p orbital.

Question Answer Marks Guidance

21 (a) (i) Protons Neutrons Electrons 1

29Si 14 16 14

(a) (ii) FIRST CHECK ANSWER ON THE ANSWER LINE 2

IF answer = 28.11 (to 2 DP) award 2 marks

(28 92.23) + (29 4.68) + (30 3.09) For 1 mark: ALLOW ECF to 2 DP if:

100 %s used with wrong isotopes ONCE

OR 28.1086 OR 28.109 OR

transposed decimal places for ONE %

= 28.11 (to 2 DP)

(b) (i) 1 NOTE: O and Cl electrons MUST be shown differently

from C electrons

Cl (e.g. expected answer)

C O IGNORE inner shells

ALLOW diagram with missing C, O or Cl symbols.

Cl

For C=O bond, ALLOW sequence × × • •

CARE: Check that lone pairs on Cl and O are included ALLOW non-bonding electrons unpaired

Cl ( 2) has 6 non-bonded electrons (3 LPs)

O has 4 non-bonded electrons (2 LPs)

Question Answer 7 Marks Guidance

(b) (ii) Shape 3 ALLOW bp for bonded pair

Trigonal planar

Number of bonded regions ALLOW 3 bonded pairs (BOD)

(C has) 3 electron (dense) regions OR 3 sigma bonds

OR 3 bonding regions OR 2 bonded pairs and 1 double bond

OR 4 bonded pairs including a double bond

IGNORE bonded atoms

IGNORE just 3 bonds

Electron pair repulsion (Seen anywhere)

electron pairs/bonded pairs/bonded regions repel ALLOW alternative phrases/words for repel

OR e.g. ‘push apart’

electron pairs move as far apart as possible

OR IGNORE electrons repel (pairs needed)

bonds repel

DO NOT ALLOW atoms repel

(c) Highest energy electron(s) in a p orbital/p sub-shell 1 ALLOW outer electron(s) in a p orbital/sub-shell

BUT IGNORE p shell

ALLOW electron configuration ends in p

OR the last electron is in a p orbital

ALLOW valence electron(s) in p orbital/sub-shell

Total 8

How to answer it

Isotopes, Mass Spectrometry, and VSEPR Theory

What this question tests

This question assesses foundational AS Chemistry topics: determining subatomic particle numbers from isotope symbols, calculating relative atomic mass from mass spectrometry data, drawing covalent dot-and-cross diagrams, predicting molecular shapes using electron pair repulsion theory (VSEPR), and identifying electronic configurations of p-block elements.

Question 21 (a) (i)

Atomic Structure of 30Si

✅ Correct Answer

Protons: 14

Neutrons: 16

Electrons: 14

Marks: 1 mark

💡 Key Knowledge

  • Atomic number (proton number, bottom left of symbol) = 14 for silicon.
  • Mass number (top left) = 30.
  • Neutrons = Mass Number minus Proton Number (30 - 14 = 16).
  • Neutral atom: electrons equal protons.
Question 21 (a) (ii)

Relative Atomic Mass Calculation

📐 Step-by-Step Calculation

  1. Identify formula: Ar = Σ (isotopic mass × percentage abundance) / 100
  2. Substitute values: ((28 × 92.23) + (29 × 4.68) + (30 × 3.09)) / 100
  3. Calculate numerator: 2582.44 + 135.72 + 92.70 = 2810.86
  4. Divide by 100: 28.1086
  5. Apply rounding: 28.11 (to 2 decimal places as requested)

❌ Common Errors & Examiner Warnings

  • Failing to give the answer to the requested two decimal places loses accuracy marks.
  • Using incorrect rounding (e.g., rounding 28.1086 to 28.1 or 28.11 without checking the third decimal digit).
Marks: 2 marks (1 for correct calculation/ECF, 1 for correct final answer to 2 DP).
Question 21 (b) (i)

Dot-and-Cross Diagram for Phosgene (COCl₂)

🧠 Exam Technique & Diagram Requirements

  • Use different symbols (e.g., dots for carbon electrons, crosses for chlorine and oxygen electrons).
  • Show the double bond between Carbon and Oxygen (4 shared electrons total).
  • Show single covalent bonds between Carbon and each Chlorine atom (2 shared electrons per bond).
  • Crucial: Do not forget non-bonding lone pairs! Each Cl atom needs 3 lone pairs (6 electrons), and the O atom needs 2 lone pairs (4 electrons).

❌ Common Errors

  • Omitting lone pairs on the outer atoms (Cl and O). Examiners heavily penalise incomplete outer shells.
  • Using the same symbol for all electrons, making it impossible to trace bond origins.
Marks: 1 mark
Question 21 (b) (ii)

Shape of Phosgene Molecule and Explanation

✅ Correct Answer

Name of shape: Trigonal planar

Explanation:

  1. Central carbon atom has 3 electron-dense regions (or 3 bonding regions / 1 double bond and 2 single bonds).
  2. Electron pairs repel each other as far apart as possible.

💡 Key Knowledge & VSEPR Rules

  • Count regions of electron density around the central atom: a double bond counts as one region of electron density.
  • Three bonding regions with zero lone pairs on the central atom always results in a trigonal planar geometry with ~120° bond angles.
  • Always state that electron pairs/regions repel to get the final mark point in shape explanations. Never state that "atoms repel".
Marks: 3 marks (1 for shape name, 1 for number of bonded regions around C, 1 for electron pair repulsion statement).
Question 21 (c)

Classification of p-Block Elements

✅ Correct Answer

Their highest energy electron(s) occupy a p orbital (or p sub-shell).

🧠 Examiner Guidance

  • Keep explanations direct and scientifically rigorous. Avoid colloquialisms like "they live in the p block" or "their p shell is filling".
  • Acceptable phrasing includes: "valence electron is in a p orbital", "outer electron configuration ends in p", or "the last electron enters a p sub-shell".
Marks: 1 mark

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.