Dot and Cross Diagrams

Dot and cross diagrams are used to show the arrangement of electrons in atoms and ions (and in some other types of particle that we will learn about later in the course, such as molecules and ionic compounds).

On this page, we will learn how to draw a dot and cross diagram of atoms and ions. Dot and cross diagrams for other types of particle will be covered later in the course.

Dot and cross diagrams of atoms

A dot and cross diagram for an atom is draw as follows:

• The nucleus is represented by the chemical symbol of the element.
• The electron shells are represented by circles drawn around the nucleus. Only the shells which have electrons in them are drawn.
• The electrons are represented by drawing either dots or crosses on the shells. A diagram for a single atom should either use all dots or all crosses.

Here is a dot and cross diagram of a chlorine atom using crosses:

A dot and cross diagram of a chlorine atom.

There are standard positions on the shells at which the electrons are usually drawn. The electrons are usually drawn at the four points of a cross with up to two electrons being drawn at each point.

Here is a dot and cross diagram for an aluminium atom, this time using dots instead of crosses:

A dot and cross diagram of an aluminium atom.

Dot and cross diagrams of ions

Dot and cross diagrams for ions are very similar to the ones for ions. However, there are a couple of important differences.

Firstly, when drawing a dot and cross diagram for an ion we need to show the ion's relative charge. The way that we do this is by drawing square brackets around the ion and writing the charge in the upper right hand corner. For example, the dot and cross diagram for a sodium ion, Na⁺, is shown below:

A dot and cross diagram of a sodium ion.

The second difference only applies to anions (negatively charged ions). We have learnt previously that an anion can be formed by an atom gaining electrons. When this happens, the anion that is formed contains some electrons that were in the original atom and other electrons that were gained in the process of becoming an ion.

Whenever we draw a dot and cross diagram of an anion, we use a mixture of dots and crosses to show how the anion could have been formed from an atom. We use one symbol (either dots or crosses) to represent the electrons from the original atom and the other symbol to represent the newly gained electrons. It does not matter which symbol is used for which.

For example, the dot and cross diagram below is for an oxide ion (O^2-). The crosses represent the 8 electrons that you would find in an oxygen atom. The dots represent the 2 electrons that would be gained by an oxygen atom to form an oxide ion.

A dot and cross diagram of an oxide ion.

We don't need to use a mixture of dots and crosses when drawing cations (positively charged ions). This is because an atom becomes a cation by losing electrons rather than gaining them, meaning that the cation only contains electrons that were in the original atom.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.