Topic 3 of 3
Structure and evidence
Connect bonds to reactivity and measured properties.
A-Level 8873, revised syllabus (2026-2027)
Compare the bond you are changing
Bond length, strength and polarity explain different aspects of reactivity.
Bond length is the equilibrium distance between the nuclei of two bonded atoms. Bond energy is the energy required to break one mole of a specified covalent bond in gaseous species; tabulated average bond energies average over molecular environments. Bond breaking is endothermic. For similar bonds, greater length usually means poorer overlap and a weaker bond.
Compare C-Cl, C-Br and C-I: the halogen atom becomes larger down the group, so the bond becomes longer and generally weaker. Breaking C-I is easier than breaking C-Cl, despite C-Cl being more polar. Polarity instead identifies electron-poor and electron-rich sites, which influences how a reagent approaches and interacts with a molecule.
Turn structures into properties, then reverse the argument
Name the particles, attraction and charge carriers before drawing a conclusion.
| Solid | Particles and structure | Properties explained |
|---|---|---|
| NaCl; MgO | Giant ionic lattice; opposite ions attract. MgO contains 2+ and 2- ions. | High melting points; fixed ions do not conduct in the solid; mobile ions conduct when molten. MgO has stronger attractions than NaCl. |
| Iodine | Discrete I2 molecules in a molecular lattice. | Comparatively low melting point: overcome intermolecular forces, not I-I bonds. No mobile charge carriers. |
| Diamond | Each carbon bonds to four others in a three-dimensional covalent network. | Hard, very high melting temperature; no mobile charged particles. |
| Graphite | Each carbon bonds to three others in a sheet; electrons are delocalised. Weaker attractions act between sheets. | High melting temperature; sheets slide, giving softness; delocalised electrons conduct. |
| Ice | Discrete water molecules joined into an open hydrogen-bonded lattice. | Less dense than liquid water; melting disrupts hydrogen bonding, not O-H bonds. |
| Copper | Positive ions in a lattice with delocalised electrons. | Conducts when solid and molten; layers can slide while metallic attraction remains. |
For melting, identify the attraction that must be overcome. For conductivity, identify mobile charged particles. For solubility, consider whether new solute-solvent attractions can compensate for attractions disrupted in both substances; "ionic" is not a guarantee of water solubility. For malleability, explain whether a displaced layer remains held together or brings like charges into unfavourable proximity.
Worked example
Use two measurements together
Solid X has a high melting point. It is an electrical insulator as a solid, but its melt conducts. Suggest a structure.
- A high melting point indicates strong attractions, but is consistent with several structures.
- The molten sample has mobile charged particles.
- An ionic lattice supplies fixed ions in the solid and mobile ions in the melt.
- A metal would usually conduct as a solid; a simple molecular solid would usually melt much lower.
A giant ionic structure best fits both observations. Do not claim a specific compound from this evidence alone.
Worked example
Distinguish two carbon structures
Two carbon solids both withstand high temperatures. Only one conducts electricity and leaves a slippery mark. Identify them.
- Conductivity implies mobile delocalised electrons: graphite.
- The slippery mark is consistent with graphite sheets sliding.
- The other can be diamond: a rigid 3-D network with all four outer electrons involved in localised bonds.
Graphite is the conducting, slippery solid; diamond is the non-conducting network. High melting temperature alone does not distinguish them.