Chemical Bonding and Structure
Chemical bonding and structure
This domain tests ionic, covalent and metallic bonding and how structure explains melting point, solubility, conductivity and malleability. GCE O-Level questions often ask you to compare substances using bonding and particles.
Core rules
- Ionic bonding involves transfer of electrons from a metal to a non-metal, forming a lattice of oppositely charged ions. Ionic compounds have high melting points, often dissolve in water, and conduct when molten or aqueous because ions are mobile.
- Covalent bonding involves sharing electron pairs. Simple molecular substances have weak intermolecular forces, so they have low melting points and do not conduct. Giant covalent substances such as diamond and silicon dioxide have high melting points; graphite conducts because it has delocalised electrons between layers.
- Metallic bonding is a lattice of positive ions in a sea of delocalised electrons. Metals conduct in solid and liquid states, are malleable and ductile because layers can slide without breaking bonds.
- Dot-and-cross diagrams must show outer electrons only and correct charges for ions.
Worked example
Explain why solid sodium chloride does not conduct electricity, but molten sodium chloride does.
Reasoning: in solid NaCl, Na+ and Cl- ions are held in fixed positions in the lattice. The ions can vibrate but cannot move through the solid, so there is no mobile charge. When melted, the lattice breaks down and the ions become free to move. Mobile ions carry charge, so molten NaCl conducts. The same applies to aqueous NaCl, where ions are free in solution. Diamond does not conduct because its outer electrons are localised in covalent bonds; graphite conducts because each carbon has one delocalised electron.
Traps that cost marks
Do not describe ionic compounds as molecules. Do not say all covalent substances have low melting points; diamond and sand are giant covalent. Metals do not conduct by moving ions. Solid ionic compounds do not conduct. In dot-and-cross diagrams, do not show inner shells unless asked.
Key points
- Ionic bonding gives lattices of ions and conducts when molten or aqueous.
- Simple covalent molecules have weak forces between molecules.
- Giant covalent structures have high melting points.
- Graphite conducts due to delocalised electrons.
- Metals conduct via delocalised electrons and are malleable.