Bonding Models and Representing Structures — AQA GCSE Chemistry
Chemists use different diagrams and models to represent bonding and structures, each with strengths and limitations.
Ways of representing structures
- Dot-and-cross diagrams — show how electrons are shared (covalent) or transferred (ionic), and which atom each electron came from. They do not show the relative sizes of atoms or the shape of the molecule.
- Displayed (2D) formulae — show each covalent bond as a line (e.g. H–O–H). Quick to draw, but do not show the 3D shape.
- Ball-and-stick models (3D) — show the arrangement and angles of bonds, giving a sense of shape, but exaggerate the space between atoms and the size of the "sticks".
- Space-filling models — show the relative sizes of atoms and how they pack, but hide the bonds inside.
Strengths and limitations
No single model is perfect; each shows some features and hides others. For example, a dot-and-cross diagram shows the electrons but not the shape, while a ball-and-stick model shows the shape but not the electrons.
Giant structures
For giant structures (ionic, covalent or metallic), diagrams show only a small part of a structure that actually contains a very large number of atoms or ions.
Exam tips
- Match each representation to what it shows and what it doesn't.
- Dot-and-cross = electrons (and their origin); ball-and-stick = shape/angles.
- All models have limitations — be able to state them.
- Giant-structure diagrams show only a repeating part of a much larger lattice.