Drawing dot and cross diagrams

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

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.

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, which leaves two electrons that do not take part in bonding.

Example of a complete dot and cross diagramThe complete dot and cross diagram for ammonia
Question

Draw a dot and cross diagram for methane, CH4.

Dot and cross diagram of methane.