Modelling molecules

There are different ways of representing small covalent molecules, including dot and cross diagrams, structural formulae and three-dimensional structures.

Structural formulae

A structural formula shows the bonds between the atoms in a molecule, or in a small section of a large polymer molecule or a giant covalent structure. In these diagrams:

  • each atom is shown by its chemical symbol
  • each covalent bond is shown as a straight line
Covalent structure of ammonia.

For example, an ammonia molecule has three covalent bonds. The diagram shows its structure.

Three-dimensional structures

The structure of a small molecule can also be shown as a three-dimensional ball-and-stick model. These models show how the atoms and bonds are arranged in space. The diagram shows a ball-and-stick model of ammonia.

Ball and stick diagram of ammonia.
Question

Give one advantage and one limitation of using a structural formula to represent a molecule.

The structural formula shows the bonds between the atoms, but it does not show which atoms the electrons in the bonds have come from.

Dot and cross diagrams

A dot and cross diagram can show the bonding in a small molecule:

  • the outer shell of each atom is drawn as a circle
  • circles overlap where there is a covalent bond
  • electrons from one atom are drawn as dots, and electrons from another atom as crosses

For example, a hydrogen molecule, H2, forms when two hydrogen atoms each share their outer electron.

Dot and cross diagram of a hydrogen bondingA dot and cross diagram to show the bonding in hydrogen

An ammonia molecule, NH3, forms when one nitrogen atom shares its outer electrons with three hydrogen atoms. There are two types of dot and cross diagram - one without circles, and one with.

Dot and cross diagram of neumonia.These dot and cross diagrams show the bonding in ammonia
Question

Give one advantage and one limitation of using a dot and cross diagram to represent a molecule.

A dot and cross diagram shows the pairs of outer electrons, and which atoms they have come from. It does not show how the atoms are arranged in space.

How many bonds?

Atoms form covalent bonds by sharing electrons to get a full outer shell. This means that the number of covalent bonds an atom can form is the same as the number of electrons needed to get a full outer shell. For most elements, a full outer shell is eight electrons.

The table below shows the number of bonds formed by elements in groups 4 to 7.

GroupExampleElectronsCovalent bonds
4Carbon, C44
5Nitrogen, N53
6Oxygen, O62
7Chlorine, Cl71

Hydrogen atoms only have one electron and form one covalent bond as they only need one more for a full outer shell.

Question

Draw a dot and cross diagram for methane, CH4.

Dot and cross diagram of methane.