AQA GCSE Combined Science: Trilogy Chemistry Paper 1 (Foundation), 2023: Question 3

13 marks · Standard Demand difficulty · Short Answer

Explain properties of compounds using structure and bonding, covering ionic, covalent, and polymer structures.

Practise this question

Question

A multi-part chemistry exam question on structure and bonding. It includes a dot and cross diagram showing the reaction between potassium and chlorine atoms to form ions. There are three grid diagrams representing possible ionic lattice arrangements of positive and negative ions. A table lists valence electrons for hydrogen (1) and oxygen (6), followed by an incomplete dot and cross diagram for a water molecule. A ball-and-stick model represents the giant covalent structure of silica. Finally, a diagram shows two polymer chains and a line graph plots melting point against the length of the polymer molecule.
Question text

03 Structure and bonding is used to explain properties of compounds.

Metal atoms react with non-metal atoms to form ions.

03.1 Which group of elements does not form ions?

[1 mark]

Tick ( ) one box.

Alkali metals

Halogens

Noble gases 12

03.2 Potassium reacts with chlorine to produce potassium chloride (KCl).

Figure 6 shows what happens to the electrons in the outer shells when

a potassium atom reacts with a chlorine atom.

The dots (o) and crosses (×) represent electrons.

Figure 6

Describe what happens when a potassium atom reacts with a chlorine atom to

produce potassium chloride.

Answer in terms of electrons.

[4 marks]

*11* 13

03.3 In solid ionic compounds, oppositely charged ions attract to form a giant structure.

Which structure represents the arrangement of ions in solid potassium chloride?

[1 mark]

Tick ( ) one box.

Non-metal atoms share electrons to form covalent bonds.

03.4 Water (H2O) is a covalent molecule.

Table 2 shows the number of electrons in the outer shells of hydrogen atoms and

of oxygen atoms.

Table 2

Element Number of electrons in the

outer shell of an atom

Hydrogen 1

Oxygen 6

Figure 7 shows part of a dot and cross diagram for a molecule of water.

Complete the dot and cross diagram.

You should only show electrons in the outer shells.

[2 marks]

*13* Figure 7

03.5 Silica has a giant covalent structure.

Figure 8 represents the structure of silica.

Figure 8

*14* Determine the ratio of silicon (Si) atoms to oxygen (O) atoms in silica.

Use Figure 8.

[1 mark]

Si : O

03.6 Polymers have very large molecules.

Figure 9 represents part of the structure of a polymer.

Figure 9

What holds polymer molecule 1 and polymer molecule 2 together in a polymer?

[1 mark]

Tick ( ) one box.

Covalent bonds

Electrostatic attraction between ions

Weak intermolecular forces 16

03.7 Figure 10 shows the melting point of a polymer as the length of the

polymer molecule increases.

Figure 10

Describe the trend shown in Figure 10.

[3 marks]

Mark scheme

Show the mark scheme The mark scheme for question 3 provides the following answers: 3.1 is 'noble gases'. 3.2 requires describing potassium atoms losing one electron and chlorine atoms gaining one electron to form ions with full outer shells. 3.3 identifies the correct alternating positive and negative ion lattice. 3.4 shows the completed water dot and cross diagram with one shared pair in each overlap and four non-bonding electrons on oxygen. 3.5 gives the ratio 1 Si : 2 O. 3.6 identifies 'weak intermolecular forces'. 3.7 describes the trend where melting point increases as polymer length increases and then levels off.

Question 3

AO /

Question Answers Extra information Mark

Spec. Ref.

03.1 noble gases 1 AO1

5.2.1.2

AO /

Spec. Ref.

03.2 potassium (atoms) lose 1 AO2

electrons

chlorine (atoms) gain electrons 1 AO2

reference to one electron 1 AO2

any one from: 1 AO1

• to form ions

• to form full outer shell(s) 5.2.1.2

• to form full energy level(s)

allow to form a noble gas

structure

AO /

Spec. Ref.

03.3 1 AO2

5.2.1.3

AO /

Spec. Ref.

03.4 allow any combination of circles, AO1

dots, crosses, e(–) for electrons 5.2.1.1

5.2.1.4

one shared pair in each overlap 1

four non-bonding electrons in do not accept extra electron(s) 1

outer shell of oxygen on outer shell of hydrogen

ignore any inner shell electrons

the diagram below scores 2

marks

AO /

Spec. Ref.

03.5 1 Si : 2 O allow 6 Si : 12 O 1 AO3

5.2.1.4

AO /

Spec. Ref.

03.6 weak intermolecular forces 1 AO1

5.2.2.5

AO /

Spec. Ref.

03.7 ignore references to boiling AO3

point 5.2.2.4

5.2.2.5

the melting point increases as allow 1 mark for melting point 2

the length of the polymer increases

molecule increases

(then the) melting point levels off allow (then the) melting point 1

12 (as the polymer molecule length becomes constant (as the

increases) polymer molecule length

increases)

Total Question 3 13

How to answer it

Bonding, Structure, and Properties

What this question tests

This question assesses your understanding of how atoms achieve stability through bonding. You need to recall the properties of Noble Gases, describe electron transfer in ionic bonding, complete covalent dot-and-cross diagrams, identify giant structures, and interpret graphical data regarding polymer properties.

Part 03.1

Ion Formation

Correct Answer

Noble gases

Key Knowledge

Noble gases (Group 0) have a full outer shell of electrons. This makes them very stable and unreactive, so they do not need to lose or gain electrons to form ions.

Part 03.2

Describing Ionic Bonding

Potassium + Chlorine → Potassium Chloride

Model Answer (4 Marks)

  • Potassium atoms lose electrons [1 mark]
  • Chlorine atoms gain electrons [1 mark]
  • Reference to one electron being transferred [1 mark]
  • To form ions OR to form full outer shells / noble gas structures [1 mark]

Exam Technique

When a question says "in terms of electrons," you must state which atom loses them and which atom gains them. Don't just say "they swap electrons."

Common Errors

Students often forget to mention the number of electrons transferred. Since Potassium is in Group 1 and Chlorine is in Group 7, exactly 1 electron moves from K to Cl.

Part 03.3

Ionic Lattice Structure

Correct Answer

The third box (the 3x3 grid of alternating charges).

Key Knowledge

Ionic compounds form a giant ionic lattice. This is a regular repeating pattern where every positive ion is surrounded by negative ions, and vice versa. This maximizes the electrostatic attraction.

Part 03.4

Covalent Dot and Cross Diagrams

Water (H₂O)

How to draw the completed diagram:
1. In the left overlap (H-O): Draw one dot and one cross.
2. In the right overlap (O-H): Draw one dot and one cross.
3. On the Oxygen atom (outside the overlaps): Draw 4 more electrons (usually as crosses) to show the remaining "lone pairs."

Mark Breakdown

  • 1 Mark: One shared pair in each overlap.
  • 1 Mark: Four non-bonding electrons on the Oxygen atom.

Common Errors

Do not add extra electrons to the Hydrogen atoms. Hydrogen only has one shell which is full with 2 electrons (the shared pair).

Part 03.5

Giant Covalent Ratios

Step-by-Step Calculation

  1. Look at the key: Si is the larger grey circle, O is the smaller white circle.
  2. In the structure of Silica (SiO₂), each Silicon atom is bonded to 4 Oxygen atoms, but each Oxygen is shared between 2 Silicons.
  3. The simplest ratio of atoms in the whole structure is 1 Si : 2 O.
Part 03.6 & 03.7

Polymers and Trends

Forces in Polymers (03.6)

Polymer chains are held together by weak intermolecular forces. While the bonds inside the chain are strong covalent bonds, the forces between the molecules are what determine the melting point.

Describing the Trend (03.7)

To get all 3 marks, you must describe the whole shape of the graph:

  • As the length of the polymer molecule increases, the melting point increases. [2 marks]
  • The melting point then levels off or becomes constant. [1 mark]

Examiner Tip

When describing a graph that curves, always look for the "turning point." If the line stops going up and goes flat, you must mention that it "levels off" to get the final mark.

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 Combined Science: Trilogy examination, Chemistry Paper 1 (Foundation), 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.