IGCSE Chemistry (0620)

Giant and simple structures and their properties

What this topic tests, the mistakes that cost marks, and one exam-style question worked through step by step.

Key ideas and common mistakes

Simple molecular substances have low melting and boiling points due to weak intermolecular forces, and do not conduct electricity because their molecules lack free ions or delocalised electrons.
Common mistake: Students often confuse these with properties of ionic or giant covalent substances.
Distinguish between diamond and graphite by the number of covalent bonds each carbon atom forms.
Common mistake: Diamond has four bonds (tetrahedral), while graphite has three (planar layers); confusing the two leads to the wrong structure.
To identify if a substance is a gas at a given temperature, compare the temperature to its boiling point.
Common mistake: If the boiling point is below the temperature, the substance is a gas; students often confuse this with the melting point or pick the lowest boiling point above the temperature.
A giant covalent lattice is a continuous network of atoms joined by covalent bonds throughout, so pick a substance like silicon(IV) oxide, silicon carbide or graphite, not a small molecule or ionic compound.
The weak attractions that must be overcome to boil a simple molecular substance are the forces between molecules (intermolecular forces), not the strong bonds within each molecule.
Common mistake: Confusing these leads to the wrong answer.
To identify a compound with a giant covalent structure, distinguish between elements and compounds, and recognize that not all giant covalent structures are compounds.

Give two physical properties of iodine that are typical of a simple molecular substance.

Try it first, then show the working

Answer: It has a low melting point and boiling point, and it does not conduct electricity in any state.

22 exam-style questions on Giant and simple structures and their properties, each marked to the scheme.