Bonding and Structure: IGCSE Chemistry Exam Guide

Bonding and structure is the topic where understanding matters more than memory. A student who understands why ionic compounds have high melting points can answer any property question the exam throws, while a student who memorised “NaCl has a high melting point” is stuck the moment the question asks about a different ionic compound. The full content is under atoms, elements and compounds; this guide focuses on turning that understanding into exam marks.

The three structures and their properties

The exam always links structure to properties. Know these three patterns and you can predict the properties of any substance from its bonding type.

Giant ionic structures

Examples: NaCl, MgO, CaF2

Structure: Positive and negative ions arranged in a regular lattice, held by strong electrostatic forces of attraction between oppositely charged ions in all directions.

Properties and the examiner’s expected explanation:

• High melting point: Strong electrostatic forces between ions require a large amount of energy to overcome.
• Conducts electricity when molten or dissolved, not when solid: In the solid, ions are in fixed positions and cannot move. When molten or dissolved in water, ions are free to move and carry charge.
• Often soluble in water: Water molecules can surround and separate the ions.
• Brittle: When a force shifts the layers, ions of the same charge come next to each other and repel, causing the crystal to shatter.

Simple molecular structures

Examples: H2O, CO2, CH4, I2

Structure: Strong covalent bonds within each molecule; weak intermolecular forces between molecules.

Properties and the expected explanation:

• Low melting and boiling points: Only the weak intermolecular forces need to be overcome to separate the molecules (the covalent bonds within the molecule do NOT break when it melts or boils).
• Do not conduct electricity: No ions and no free electrons to carry charge.
• Often insoluble in water (except small polar molecules).

The critical error students make: saying “the covalent bonds are weak” or “the covalent bonds break when it melts.” The covalent bonds are strong; it is the forces BETWEEN the molecules that are weak.

Giant covalent structures

Examples: Diamond, graphite, silicon dioxide

Structure: Every atom is bonded to neighbouring atoms by strong covalent bonds in a continuous network throughout the structure.

Properties:

• Very high melting points: Many strong covalent bonds must be broken.
• Very hard (diamond) or layered (graphite).
• Do not conduct electricity (diamond) because all electrons are used in bonding. Graphite conducts because each carbon atom forms only three covalent bonds; the fourth electron is delocalised and free to move along the layers.

Metallic bonding

Structure: Positive metal ions in a regular arrangement surrounded by a sea of delocalised electrons.

Properties:

• Good conductors of electricity: Delocalised electrons are free to move and carry charge.
• Malleable and ductile: Layers of ions can slide over each other without breaking the metallic bond because the sea of electrons moves with them.
• High melting points (generally): Strong attraction between the positive ions and the sea of delocalised electrons.

Dot-and-cross diagrams: where marks are won and lost

Ionic bonding

1. Draw the outer shell electrons of each atom (dots for one element, crosses for the other).
2. Show the transfer: the metal atom loses electrons, the non-metal gains them.
3. Draw each ion in square brackets with the correct charge.

Common errors:

• Forgetting square brackets around the ions.
• Wrong charges (Mg2+ not Mg+, because magnesium loses two electrons).
• Drawing inner shell electrons when only the outer shell is needed.
• Not showing the transferred electrons on the correct ion.

Covalent bonding

1. Draw the outer shell electrons of each atom.
2. Show shared pairs in the overlapping region between atoms.
3. Each shared pair is one covalent bond.

Common errors:

• Drawing too many or too few shared pairs (oxygen needs two shared pairs, nitrogen needs three).
• Not showing lone pairs when the question asks for a full dot-and-cross diagram.
• Drawing bonds between atoms that should not be bonded (e.g., both hydrogens bonding to each other in water instead of to the oxygen).

The property-prediction question

Paper 4 frequently gives you a substance and asks you to predict and explain its properties from its bonding type. The method:

1. Identify the bonding type from the formula (metal + non-metal = ionic; non-metal + non-metal = covalent; pure metal = metallic).
2. Identify the structure type (ionic = giant ionic; small covalent molecules = simple molecular; C as diamond/graphite or SiO2 = giant covalent).
3. State the properties that follow from that structure, using the precise explanations above.

Worked exam question

Q (Paper 4): Explain why diamond has a very high melting point but methane has a very low boiling point. Both contain only covalent bonds. (4 marks)

Model answer: Diamond has a giant covalent structure in which every carbon atom is bonded to four other carbon atoms by strong covalent bonds throughout the structure (1). A very large amount of energy is needed to break the many strong covalent bonds, so the melting point is very high (1). Methane has a simple molecular structure with strong covalent bonds within each molecule but only weak intermolecular forces between the molecules (1). Only a small amount of energy is needed to overcome the weak intermolecular forces, so the boiling point is very low (1).

The key to full marks: naming the structure type (giant covalent vs simple molecular) and specifying WHAT is being overcome (covalent bonds throughout the structure vs weak intermolecular forces). Writing “diamond has strong bonds” and “methane has weak bonds” without distinguishing intramolecular from intermolecular earns at best 2 out of 4.

Bonding and structure is a topic where understanding the reasoning unlocks marks across the paper. If dot-and-cross diagrams or property explanations are a persistent weak area, a trial lesson can identify the specific misconception and correct it.