Drawing structures

Instead of trying to remember lots of different dot and cross diagrams, it may help to understand how to draw them using given information.

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

Note: Hydrogen atoms only have one electron and form one covalent bond as they only need one more electron for a full outer shell. This is because the first electron shell is filled by only two electrons.

How many circles?

To work out how many circles to draw for a simple molecular substance and how to label them, look at the formula. For example, the formula for ammonia is NH3. For this, draw four circles, one labelled N and three labelled H. Each of the three H circles overlaps the N circle.

An example of how chemistry circles are drawnIt is often easiest to draw circles at 90° or 180° to each other

Nitrogen is in group 5 so it forms three covalent bonds. There are three shared spaces between the circles, so add a dot and cross to each one.

Example of an incomplete dot and cross diagramThis incomplete dot and cross diagram shows only the bonding pairs of electrons

Finally add in the non-bonding outer electrons. Nitrogen atoms have five outer electrons. Three of these are shared, leaving two non-bonding electrons. Add these to complete the diagram (hydrogen atoms only have one electron, so they have no non-bonding electrons).

The number of non-bonding electrons is almost always a multiple of two, and it is usually much clearer (and easier to check when drawing a structure) to arrange the non-bonding electrons in pairs, to match the pairs of electrons in the covalent bonds.

Example of a complete dot and cross diagramThe complete dot and cross diagram for ammonia showing the non-bonding pair of electrons

Double and triple bonds

Some molecules contain a double bond, which consists of two shared pairs of electrons. For example, oxygen molecules consist of two oxygen atoms joined together. Oxygen atoms can form two covalent bonds, so to link the two oxygen atoms together, a double bond forms between them.

Table of oxygen and carbon dioxide dot and cross diagrams

Nitrogen molecules consist of two nitrogen atoms joined together. Nitrogen atoms can form three covalent bonds, so a triple bond forms between them. The structure of nitrogen is N≡N, showing that it has three shared pairs of electrons.