What you'll learn
Chemists use several different ways of drawing molecules and structures, and each one shows some information clearly while hiding other details. For AQA GCSE Chemistry you need to understand the main bonding models — dot-and-cross diagrams, ball-and-stick models, two-dimensional structures, and three-dimensional models — and know the strengths and limitations of each. This guide explains what each representation shows, when to use it, and why no single model is perfect. By the end you should be able to choose the right model for a purpose, interpret each type, and evaluate its advantages and disadvantages in an exam answer.
Key terms and definitions
Model — A simplified way of representing something to make it easier to understand; every model has limitations.
Dot-and-cross diagram — A diagram that shows how electrons are arranged and shared or transferred in bonding, using dots for one atom's electrons and crosses for another's.
Ball-and-stick model — A three-dimensional model showing atoms as balls and bonds as sticks, giving the shape and bond angles of a molecule.
Two-dimensional (2D) structure — A flat drawing of a molecule showing atoms as symbols and bonds as lines.
Three-dimensional (3D) model — A representation that shows the true shape and arrangement of atoms in space.
Displayed formula — A 2D diagram showing every atom and every bond as a line.
Limitation — A feature that a model fails to show correctly or realistically.
Core concepts
Why we need models
Atoms and molecules are far too small to see, so chemists represent them with models. A model is a simplified picture that makes a substance easier to understand, but because it is simplified, it always leaves something out. The skill is choosing the model that shows the information you need and knowing what each one does not show.
Dot-and-cross diagrams
A dot-and-cross diagram shows how the outer electrons are arranged during bonding. One atom's electrons are drawn as dots and the other's as crosses, so you can see clearly which electrons come from which atom. For covalent bonding, the diagram shows the shared pairs of electrons; for ionic bonding, it shows electrons being transferred and the resulting charges. The strength of this model is that it shows exactly where the bonding electrons come from and how they are shared or transferred. Its limitation is that it gives no idea of the shape of the molecule, the size of the atoms, or the real position of the electrons — in reality, electrons are not fixed dots.
Two-dimensional structures
A 2D structure, or displayed formula, shows atoms as their chemical symbols joined by lines representing bonds. It is quick to draw and clearly shows which atoms are joined to which and how many bonds there are. Its limitation is that it is flat, so it does not show the real three-dimensional shape or bond angles of the molecule.
Ball-and-stick models
A ball-and-stick model represents atoms as balls and bonds as sticks. Its great strength is that it shows the three-dimensional shape of a molecule and the angles between the bonds, which a flat diagram cannot. However, it is misleading about scale: the sticks make it look as if atoms are far apart, when in reality the atoms are touching and there is no empty space or physical "stick" between them. It also does not show the electrons.
Three-dimensional models
A full 3D model, such as a space-filling model, shows the true shape and relative sizes of atoms packed together. This is the most realistic for shape and size, but it can hide the bonds inside the structure and is harder to draw. As with all models, gaining realism in one respect costs clarity in another.
Representing giant structures
The models above work well for small molecules, but giant structures — ionic lattices, metals, and giant covalent structures like diamond — need their own representations. A giant ionic structure is often drawn as a repeating lattice of alternating positive and negative ions, which shows the regular arrangement but can only display a small part of a structure that really contains billions of ions. A metal is drawn as a lattice of positive ions in a "sea" of delocalised electrons, which explains conduction but is again only a tiny sample of the whole. The key limitation shared by all these diagrams is that they show only a small, repeating portion, so you must remember the real structure continues in every direction. When answering questions, make clear that the diagram represents a repeating pattern, not the whole substance.
Comparing the models
| Model | Shows well | Main limitation |
|---|---|---|
| Dot-and-cross | Where bonding electrons come from | No shape or scale |
| 2D / displayed formula | Which atoms bond to which | Flat — no real shape |
| Ball-and-stick | 3D shape and bond angles | Wrong scale; atoms look too far apart |
| 3D space-filling | True shape and relative sizes | Hides internal bonds |
Worked examples
Example 1: Choosing a model for electrons
A student wants to show how electrons are shared in a water molecule. Which model should they use? A dot-and-cross diagram, because it shows clearly which electrons come from the oxygen and which from the hydrogens, and how the pairs are shared in the covalent bonds. A ball-and-stick model would show the shape but not the electrons.
Example 2: Evaluating a ball-and-stick model
Give one advantage and one disadvantage of a ball-and-stick model of methane. An advantage is that it shows the three-dimensional shape and the angles between the bonds. A disadvantage is that it shows the atoms as too far apart, because in reality the atoms are much closer together and there is no physical stick between them.
Example 3: Matching the model to the purpose
Which model is best for showing that a molecule is flat and which atoms are joined, but does not need to show shape in 3D? A two-dimensional displayed formula, because it shows every atom and bond clearly and is quick to draw, even though it cannot show the true 3D shape.
Example 4: Explaining why no model is perfect
Explain why chemists use several different models for the same molecule. Each model shows some features well but leaves others out: dot-and-cross shows electrons but not shape, ball-and-stick shows shape but the wrong scale, and 2D structures show connections but are flat. Using several models together gives a fuller understanding than any single one can.
Common mistakes and how to avoid them
A very common error is treating a model as if it were the real thing. Electrons are not really dots and crosses, and there are no real sticks between atoms — these are just ways of representing bonding. Always remember a model is a simplification.
Students often give a strength when asked for a limitation, or the other way round. Learn one clear strength and one clear limitation for each model so you can answer "evaluate" questions confidently.
When describing the ball-and-stick limitation, be specific: the problem is that it makes the atoms look too far apart and shows empty space that is not really there. Simply writing "it is not realistic" is too vague for the marks.
Another mistake is thinking a 2D displayed formula shows the shape of the molecule. It does not — it is flat and shows only which atoms are connected, not the real three-dimensional arrangement.
Finally, do not confuse the models: a dot-and-cross diagram is about electrons, while ball-and-stick and 3D models are about shape. Match the model to what the question is asking about.
Exam technique for "Bonding models and representing structures"
Questions here often ask you to evaluate a model — that means giving both advantages and disadvantages. Structure your answer as "one thing it shows well" and "one thing it fails to show", and be specific rather than vague.
When asked which model to use for a given purpose, decide whether the purpose is about electrons (choose dot-and-cross) or about shape (choose ball-and-stick or 3D). State your reason clearly.
A common six-mark question asks you to compare two or more representations of the same substance. Plan your answer as a small table in your head — model, strength, limitation — then write it out in full sentences. Always finish by making the point that no single model is complete, which is why several are used together.
Quick revision summary
- A model is a simplified representation; every model has strengths and limitations.
- Dot-and-cross diagrams show where bonding electrons come from and how they are shared or transferred, but not the shape or scale.
- 2D / displayed formulae show which atoms are joined and how many bonds, but are flat.
- Ball-and-stick models show 3D shape and bond angles, but wrongly make atoms look too far apart.
- 3D space-filling models show true shape and relative size, but hide internal bonds.
- Chemists use several models together because none shows everything.