OxfordAQA International GCSE Chemistry: How bonding and structure are related to the properties of substances (9202) – Revision Notes

These revision notes condense section 3.2.2 of OxfordAQA’s International GCSE Chemistry (9202) specification (Version 6.2, which applies to exams May/June 2018 onwards): how bonding and structure are related to the properties of substances, statements a to g. There are no tiers in this qualification, so all of it is for you. The fuller teaching, with worked examples, is in the study guide; to practise exam-style answers, use the practice questions. Unsure where your gaps lie? A free diagnostic takes about ten minutes.

The two questions behind every property

1. Melting and boiling: what forces must be overcome, and are they strong or weak?
2. Conducting electricity: are there charged particles, and are they free to move?

Answer both for a structure and you can explain any property in this section.

Key terms

Term	Meaning
Giant ionic lattice	Regular repeating 3D arrangement of oppositely charged ions
Electrostatic attraction	Force between opposite charges; in a lattice it acts in all directions
Simple molecule	Small group of atoms joined by covalent bonds, e.g. H₂O, NH₃
Intermolecular forces	Weak forces between separate molecules
Giant covalent structure (macromolecule)	Atoms all joined by strong covalent bonds in a continuous network
Delocalised electrons	Outer-shell electrons free to move through a whole metal structure

Summary table

Structure	Examples	Forces overcome on melting	Melting point	Solid conducts?	Liquid conducts?
Giant ionic	Sodium chloride	Many strong electrostatic attractions between ions	High	No	Yes (also when dissolved)
Simple molecular	H₂, O₂, N₂, Cl₂, HCl, H₂O, NH₃, CH₄	Weak intermolecular forces only	Relatively low	No	No
Giant covalent	Diamond, graphite, silica (SiO₂)	A vast number of covalent bonds	Very high	No (graphite: yes)	–
Metallic	Copper, iron, sodium	Strong attraction between ions and delocalised electrons	Usually high	Yes	Yes

3.2.2 a: ionic compounds have high melting points

• Structure: giant ionic lattice. In sodium chloride, every ion has six nearest neighbours of the opposite charge. The specification asks only for sodium chloride’s structure.
• Forces: strong electrostatic attraction, ion to oppositely charged ion, in all directions.
• Property: high melting and boiling points, since a large amount of energy is needed to break the many strong bonds.

Method in steps: “Explain why ionic compound X has a high melting point.”

1. Name the structure: giant ionic lattice.
2. Name the force: strong electrostatic attraction between oppositely charged ions.
3. Say there are many of them, in all directions.
4. Link to energy: overcoming them takes a lot of energy.

3.2.2 b: ionic compounds conduct when melted or dissolved

State	Ions free to move?	Conducts?
Solid	No, fixed in lattice	No
Molten	Yes	Yes
Dissolved in water	Yes	Yes

The charge carriers are ions, never electrons.

3.2.2 c, d and e: simple molecules

• c: gases, liquids or solids with relatively low melting and boiling points.
• d: melting or boiling overcomes the weak intermolecular forces between molecules. The covalent bonds are not broken. Intermolecular forces are weak compared with covalent bonds.
• e: no electrical conduction, because the molecules have no overall electric charge.

Small worked reminder. Use these values for methane: energy to boil 1 mol = 8 kJ; each C–H bond needs 413 kJ per mol to break. One CH₄ molecule has four C–H bonds, so breaking all of them needs 4 × 413 = 1652 kJ per mol. That is 1652 ÷ 8 ≈ 207 times the boiling energy, which shows boiling cannot be breaking covalent bonds.

Method in steps: state at a given temperature.

1. Below the melting point: solid.
2. Between melting point and boiling point: liquid.
3. Above the boiling point: gas.

Take care with negative numbers: −196 °C is lower than −7 °C.

3.2.2 f: giant covalent structures

• Every atom is linked to other atoms by strong covalent bonds, throughout the structure.
• Examples to know: diamond, graphite (both forms of carbon) and silicon dioxide (silica).
• Property: very high melting points, because a very large number of strong covalent bonds must be broken.
• There are no intermolecular forces to mention, because there are no separate molecules.
• Graphite conducts and is soft; those details sit in section 3.2.3.

Recognising structures from diagrams.

Clue in the diagram	Structure
Alternating + and − particles in a regular pattern	Giant ionic
Positive ions with “e⁻” or a sea of electrons between them	Metallic
Small, separate groups of bonded atoms with space between them	Simple molecular
Atoms joined by bonds that continue to the edge in every direction	Giant covalent

3.2.2 g: metals conduct heat and electricity

• Metals contain delocalised electrons that can move through the whole structure.
• Moving electrons carry charge (electrical conduction) and energy (thermal conduction).
• Conduction depends on the ability of electrons to move through the metal.

Model answer: comparing two structures

Explain why sodium chloride melts at about 801 °C but ice melts at 0 °C. A strong answer covers both substances and links each to energy:

• Sodium chloride is a giant ionic lattice. Melting it means overcoming many strong electrostatic attractions between oppositely charged ions, acting in all directions.
• Ice is made of simple H₂O molecules. Melting it overcomes only the weak intermolecular forces between molecules; the O–H covalent bonds stay intact.
• Far more energy is needed to overcome the forces in sodium chloride, so its melting point is much higher.

A one-sided answer that only describes sodium chloride is incomplete. A comparison question wants a point about each substance, plus the energy link.

Must-know distinctions

• Covalent bond vs intermolecular force. A covalent bond is inside a molecule and is strong. An intermolecular force is between molecules and is weak. Melting a simple molecular solid overcomes only the second.
• Simple molecular vs giant covalent. Both are covalent. Simple molecular substances have low melting points; giant covalent substances have very high ones. The difference is whether there are separate molecules.
• Ions vs electrons as charge carriers. Molten or dissolved ionic compounds conduct by moving ions. Metals conduct by moving electrons.
• “Strong bonds” vs “a lot of energy”. Strong forces are the cause; the high melting point follows because overcoming those forces takes a lot of energy. Write both.

Quick self-test

1. What type of force holds the ions together in sodium chloride?
2. How many chloride ions surround each sodium ion in solid sodium chloride?
3. Why does solid calcium oxide not conduct electricity?
4. Give two conditions under which an ionic compound conducts.
5. Use these values: substance X melts at −7 °C and boils at 59 °C. State its state at 25 °C.
6. Use these values: substance Y melts at −210 °C and boils at −196 °C. State its state at 25 °C.
7. Use these values: substance Z melts at 1083 °C and boils at 2567 °C, and conducts as a solid. Identify its structure type.
8. When liquid ammonia boils, which forces are overcome?
9. Use these values for oxygen: energy to boil 1 mol = 7 kJ; energy to break the O=O bonds in 1 mol of O₂ = 498 kJ. Roughly how many times larger is the bond-breaking energy?
10. Why does liquid methane not conduct electricity?
11. Name three giant covalent substances listed in the specification.
12. Which particles carry the current in a metal wire?

Answers

1. Strong electrostatic attraction between oppositely charged ions (Na⁺ and Cl⁻), acting in all directions.
2. Six.
3. Its ions are locked in place in the lattice, so none can travel to carry charge.
4. When melted (molten) and when dissolved in water.
5. 25 °C is between −7 °C and 59 °C, so liquid.
6. 25 °C is above −196 °C, so gas.
7. High melting point and conduction as a solid: metallic.
8. The weak intermolecular forces between ammonia molecules. The N–H covalent bonds are not broken.
9. 498 ÷ 7 ≈ 71 times.
10. Methane molecules have no overall electric charge, so there are no charged particles to carry a current.
11. Diamond, graphite and silicon dioxide (silica).
12. Delocalised electrons.

Where marks are usually lost

• Claiming that melting or boiling a molecular substance breaks its covalent bonds.
• Calling intermolecular forces “weak bonds” without making clear they are between molecules.
• Saying electrons move in a molten or dissolved ionic compound; the ions move.
• Explaining “high melting point” with “strong bonds” alone and never mentioning the large amount of energy needed to overcome them.
• Leaving out “in all directions” or “oppositely charged” when describing ionic bonding in a lattice.
• Describing silicon dioxide as made of SiO₂ molecules, or comparing it with carbon dioxide by “stronger intermolecular forces”.
• Claiming a solid ionic compound contains “no ions”. The ions are there; they just cannot travel.
• Misreading negative temperatures when deciding states from melting and boiling point data.
• For metals, saying “free ions” carry the current instead of delocalised electrons.

Links

• Study guide for this section
• Practice questions for this section
• Exam preparation for this course
• Every unit of this course, on the hub
• Tick-off checklist
• Ten-minute diagnostics

Official syllabus

Published by OxfordAQA: the International GCSE Chemistry (9202) specification, Version 6.2, valid for exams May/June 2018 onwards. This page follows its section 3.2.2, “How bonding and structure are related to the properties of substances”.