Structure: Models of Bonding and Structure
Structure 2.1–2.4
Every property question IB throws at you — why does this melt higher, why does that conduct here but not there — traces back to one of three bonding models: ionic, covalent, or metallic. This topic asks you to build each model from its electrostatic basics, then use it to predict and explain real behaviour: melting point, conductivity in different states, shape, solubility, malleability. The exam almost never tests the model in isolation — it tests whether you can name the right structural detail (coordination number, electron domain count, number of delocalized electrons) and explain why that detail produces the observed property. Vocabulary without mechanism gets you nowhere on the mark scheme.
Overview: The Shape of This Topic
Why three models?
A bonding model is a deliberate simplification — real electron density never comes in neat 'ionic' or 'covalent' boxes, but treating structures as (a) oppositely charged ions in a lattice, (b) atoms sharing electron pairs, or (c) cations sitting in a sea of delocalized electrons lets you predict melting point, conductivity, shape and mechanical behaviour with a handful of rules.
- Structure 2.1–2.3 build each model separately from its electrostatic basis.
- Structure 2.4 admits none of the three is ever 'pure' — bonding sits on a continuum, and the model you choose has to match the electronegativity data, not the other way round.
- Almost every exam question is really asking: which structural feature explains this property? — coordination number, lone pair count, or number of delocalized electrons.
The shape of the chapter
Command terms that decide the mark
| Command term | What it demands | AO | Mark-earning move |
|---|---|---|---|
| Compare | State similarities AND differences between the two things named. | AO3 | A list of only differences (or only similarities) caps the available marks even if every statement is correct. |
| Deduce / Predict | Use given information (charges, radii, electron domains) to reach a specific answer. | AO2 | Method marks are separate from the final-answer mark — show the reasoning step (e.g. domain count, charge product) explicitly. |
| Explain | Give the underlying reason or mechanism, not just restate what happens. | AO3 | Restating the observation ('it conducts because it's ionic') without the mechanism ('ions become mobile') scores zero. |
| Outline | Give a brief account of the main points only. | AO1 | Over-long answers waste time; 2–3 concise points usually meet the mark allocation. |
| Distinguish | Point out differences between two similar things. | AO2 | A definition of only one term, or a vague 'they're different', doesn't earn the distinguishing mark. |
Key point
Overview