Every model shows some things and hides others

1. Chemists describe particles through models, because the particles themselves are far too small to observe directly.
2. Each one is built to show a single kind of information clearly, and gives up others to do so.
3. Judging a model means asking what it was designed to show, rather than whether it is a perfect picture.
4. No single representation shows particles, electrons, forces, sizes and shapes all at once.
5. Two models used together often cover each other's gaps.

Dot-and-cross diagrams: electrons yes, shape and scale no

1. A dot-and-cross diagram shows the outer-shell electrons and which atom each one came from.
2. For a covalent molecule it shows the shared pairs, so the number of bonds is clear.
3. For an ionic compound it shows the electrons transferred and the charges that result.
4. It leaves out the nucleus and the inner-shell electrons.
5. It is flat, so it shows no three-dimensional shape and no bond angles.
6. Symbol sizes and spacings are chosen for readability, so nothing in it is drawn to scale.
7. For an ionic compound it shows one pair of ions rather than the repeating lattice.

Ball-and-stick models: shape yes, scale and electrons no

1. A ball-and-stick model shows which atoms are joined and roughly how they sit in three dimensions.
2. It gives a usable impression of the bond angles.
3. The balls are drawn far apart relative to their size, so the spacing is wrong.
4. Real atoms are not solid coloured spheres, and the colours are a labelling convention.
5. The sticks show that a bond exists but say nothing about shared or transferred electrons.
6. Atoms at the back can be hidden from any one viewing angle.
7. For an ionic compound it can show the lattice arrangement but not the charges or the attractions.

Two-dimensional and three-dimensional representations

1. A two-dimensional drawing shows which atoms are joined and how many bonds each one has.
2. It cannot show depth, so atoms nearer and further away look alike.
3. A three-dimensional model or drawing shows the spatial arrangement and the approximate bond angles.
4. It can still distort atom sizes, bond lengths and the spaces between particles.
5. One viewing angle can hide atoms or make separate bonds appear to overlap.
6. A flat drawing is not wrong merely for being flat, and its limitation is the missing depth.

Choosing and combining models

1. Use a dot-and-cross diagram when the question is about electrons, bonds or charges.
2. Use a ball-and-stick model when the question is about shape or arrangement.
3. Use a two-dimensional drawing when the question is only about which atoms are joined.
4. Use two models together when the question needs both the electrons and the shape.
5. Name the limitation that matters for the question rather than listing every limitation the model has.