Structure: Models of Bonding and Structure
Structure 2
Every question in this topic is really the same question dressed up differently: what particles are present, and what holds them together? Once you name the particles (ions, atoms, molecules, or cations-in-a-sea-of-electrons) and the force between them, melting point, conductivity, solubility and hardness all fall out logically — you don't need to memorise a separate rule for each property.
Overview: The Bonding Continuum
IB treats ionic, covalent and metallic bonding not as three separate boxes but as points on a continuum of electron sharing. Pure ionic (complete electron transfer) and pure covalent (equal sharing) are idealised limits — real bonds like Al–Cl or Li–F sit somewhere between, and HL specifically tests your ability to justify where on that spectrum a bond sits.
- Structure 2.1–2.3 build the three models (ionic, covalent, metallic); Structure 2.4 asks you to use those models to predict and explain macroscopic material properties.
- The recurring exam move: give ionic-lattice-type data (charges, radii, structure) and ask which factor controls lattice enthalpy, solubility, melting point or brittleness — it is always the same Coulombic argument.
The shape of the chapter
Command terms this topic loves
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Deduce | Reach a conclusion from the given data (radii, charges, structure) using a stated rule — show the logical chain, not just the answer. | AO3 | Each link in the reasoning chain is a mark; a bare final answer with no working loses most of the marks. |
| Explain | State the mechanism/cause AND its effect — e.g. 'higher charge → stronger electrostatic attraction → more energy needed → higher melting point'. | AO2 | Cause alone or effect alone scores half; both linked scores full. |
| Compare and contrast | State at least one similarity AND one difference explicitly. | AO2 | Answers that only give differences (or only similarities) are capped at half marks. |
| Predict | Use a trend or model to state an unmeasured outcome, with justification implied by the model used. | AO2 | A prediction with no supporting reasoning often scores zero even if the value is right. |
| Suggest | Propose a plausible explanation where more than one reasonable answer exists — used for open, less standard scenarios (e.g. LiBOB stability). | AO3 | Any chemically sound reasoning is accepted; examiners reward logic, not just the 'model' answer. |
Key point
Overview