Model Limitations

No single model shows everything. For example, a model designed to show where bonding electrons came from may be poor at showing molecular shape. Evaluating a model therefore means asking two questions:

1. What useful information does it show?
2. What important information does it omit, simplify or distort?

Dot-and-cross diagrams: concentrating on electrons

In a dot-and-cross diagram, dots and crosses distinguish electrons from different atoms. For a covalent substance, the diagram can show shared pairs of electrons and non-bonding electrons. For a binary ionic substance, it can show electron transfer and the resulting outer-shell arrangements of the ions.

This makes the model useful for explaining bonding in terms of electron sharing or transfer. However, the symbols are only labels: electrons are not literally dots or crosses, and the distinction does not mean that electrons remain different after bonding. The diagram also does not represent the movement of electrons.

Dot-and-cross diagrams usually show electron shells as flat circles. They do not show the true relative sizes of atoms or ions, realistic distances, or the three-dimensional arrangement of particles. A single pair of ions can also give a misleading impression that an ionic compound consists of separate ion pairs; in reality, an ionic solid has a giant, repeating three-dimensional lattice.

Ball-and-stick models: concentrating on arrangement

A ball-and-stick model makes connectivity and spatial arrangement easier to see. It can help a learner turn a flat drawing into a three-dimensional idea and recognise the shape of a molecule. It can also represent a small part of a giant covalent or ionic structure.

Its appearance is highly simplified. The balls are not normally the correct relative sizes, while the sticks make the particles appear much farther apart than they really are. A chemical bond is an electrostatic attraction, not a solid rod of matter. Colours may help distinguish elements but are conventions rather than the real colours of individual atoms.

Ball-and-stick models also reveal little about electron transfer, shared electron pairs or electron movement. When a model shows a giant lattice, it can display only a tiny fragment: the real pattern continues in three dimensions through an enormous number of particles.

Two-dimensional representations

Displayed formulae and diagrams on paper are two-dimensional. They are quick to draw and can clearly show which atoms are joined and, where relevant, whether bonds are single, double or triple. They are useful when connectivity matters more than shape.

Their main limitation is that a flat page has no actual depth. A drawing such as a simple cross-shaped displayed formula may make bonds appear to lie in one plane even when the molecule has a three-dimensional shape. Particles can also hide behind one another when a three-dimensional structure is projected onto a flat page. Unless extra perspective conventions are provided, bond angles and spatial positions can be unclear or misleading.

Three-dimensional representations

A three-dimensional drawing or physical model can show how particles are arranged in space. This is especially helpful for molecular shapes and for showing that an ionic or giant covalent lattice extends in three dimensions rather than being a single flat sheet.

Even a three-dimensional representation remains a model. It may not be to scale, particles can obscure those behind them, and a drawing can show only one viewpoint. Giant structures must be represented by a limited section rather than in full. Detailed three-dimensional drawings are also slower and more difficult to produce than simple two-dimensional formulae.

Translating between 2D and 3D

To interpret a flat picture of a three-dimensional structure, look for perspective clues: particles drawn partly behind others, bonds shown at angles and a repeating pattern continuing backwards as well as sideways. When converting a 3D model to a 2D drawing, preserve the important information—such as which particles are connected and whether the pattern repeats—but recognise that some depth information will be lost.

This is why different representations may be used together. A dot-and-cross diagram can explain the electrons involved in bonding, while a ball-and-stick or three-dimensional model can communicate spatial arrangement. Using both provides a fuller account than either model alone.

Choosing and evaluating a model

The best model depends on the question. To explain which electrons are transferred or shared, choose a dot-and-cross diagram. To investigate molecular shape or the spatial arrangement of a lattice, choose a ball-and-stick or three-dimensional representation. To record connectivity quickly, a two-dimensional displayed formula may be enough.

A strong evaluation is specific. Rather than saying that a model is simply “inaccurate”, explain the mismatch: for example, “the ball-and-stick model shows the three-dimensional arrangement, but its atom sizes and bond lengths are not to scale and it does not show the bonding electrons.”