Interactions Over Small and Large Distances: Structure and Bonding — AQA Combined Science: Synergy
This topic covers the three types of chemical bonding and how structure explains the properties of substances.
Types of chemical bonding
Atoms bond to achieve a full outer shell. The type depends on the elements:
- Ionic — metal + non-metal.
- Covalent — non-metal + non-metal.
- Metallic — metal atoms.
Ionic bonding
Electrons are transferred from the metal (forming a positive ion) to the non-metal (forming a negative ion). The oppositely charged ions are held by strong electrostatic forces in a giant ionic lattice. Dot-and-cross diagrams show where electrons come from.
Properties of ionic compounds
- High melting and boiling points — strong forces need lots of energy to break.
- Conduct electricity when molten or dissolved (ions free to move) but not when solid.
Covalent bonding
Atoms share pairs of electrons. Covalent substances form:
- Small molecules (e.g. H₂O, CO₂) — strong bonds within molecules but weak intermolecular forces, giving low melting/boiling points; do not conduct.
- Giant covalent structures (e.g. diamond, graphite, silicon dioxide) — huge networks with very high melting points.
- Polymers — large molecules, usually solid.
Properties of substances with covalent bonding
- Diamond — each carbon bonded to four others; very hard; does not conduct.
- Graphite — each carbon bonded to three; layers slide (soft lubricant); has delocalised electrons so it conducts.
- Graphene — a single layer; strong and conducting.
Metallic bonding
Metal atoms form a giant lattice of positive ions in a sea of delocalised electrons, held by strong electrostatic attraction.
Properties of metals
- High melting/boiling points — strong metallic bonds.
- Good conductors of heat and electricity — delocalised electrons carry charge and energy.
- Malleable — layers of ions can slide.
- Alloys are harder than pure metals because different-sized atoms distort the layers, stopping them sliding.
Exam tips
- Match structure to properties — explain every property using bonding.
- Ionic compounds conduct only when molten or dissolved (ions free to move).
- Simple molecules have low melting points due to weak intermolecular forces (not weak covalent bonds).
- Explain metal conductivity and malleability, and alloy hardness, using delocalised electrons and sliding layers.