Chemical Bonding and Structure
This domain tests how atoms combine and how the resulting structure explains melting point, solubility, conductivity and strength. You must distinguish ionic, covalent and metallic bonding and link each to properties.
Types of bonding
- Ionic bonding: electrons are transferred from a metal to a non-metal, forming oppositely charged ions held in a giant lattice. Ionic compounds have high melting points, are usually soluble in water, and conduct electricity when molten or aqueous because the ions are free to move.
- Covalent bonding: non-metal atoms share pairs of electrons. Simple molecular substances have weak forces between molecules, so they have low melting points and do not conduct. Giant covalent substances such as diamond, graphite and silicon dioxide have very high melting points.
- Metallic bonding: a lattice of positive ions surrounded by delocalised electrons. Metals conduct electricity, are malleable and ductile.
Worked example
Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. In the solid, Na⁺ and Cl⁻ ions are held in fixed positions in the lattice, so there are no mobile charged particles. When melted, the lattice breaks down and the ions become free to move. Moving ions carry charge, so the molten liquid conducts.
Dot-and-cross diagrams
Show only outer-shell electrons. Ionic diagrams show complete transfer and square brackets with charges. Covalent diagrams show shared pairs in the overlap region. A double bond is two shared pairs.
Traps that cost marks
- Saying covalent bonds are weak. The bond is strong; the weak forces are between molecules.
- Claiming all covalent substances are gases or liquids; diamond and silicon dioxide are giant covalent solids.
- Forgetting that graphite conducts because of delocalised electrons between layers, while diamond does not.
- Drawing ionic diagrams without charges or without showing the transferred electron.