Limitations of models of ions and ionic compounds

Dot and cross diagrams

A dot and cross diagram shows:

However, it does not show how the ions are arranged in space.

Structures of a sodium ion and a choride ion.

A dot and cross diagram for sodium chloride suggests that it is made up of pairs of sodium and chloride ions. It is not.

Sodium chloride has a giant ionic lattice structure with equal numbers of sodium and chloride ions.

The ionic lattice structure of an ionic compound needs a 3D model, or a 2D representation of a 3D model.

3D construction models of ionic compounds

A 3D construction model shows how the ions are arranged in a lattice structure. These models usually use coloured balls to represent the ions. Some use sticks to show the ionic bonds.

A 3D construction model still has limitations:

  • it is not to scale
  • it gives no information about the forces of attraction between the ions, or the movement of electrons to form the ions
3D construction model showing the arrangement of ions in a lattice structure.
This model does not show the relative sizes of the ions

3D diagrams of ionic compounds

A 3D model may be drawn as a 3D diagram. The limitations are the same as the 3D construction model.

On a computer it may be possible to rotate this diagram to look at it from different angles. This helps to visualise the structure.

Ionic structure of sodium chloride forming a cubic latticeA three-dimensional model for the ionic lattice in sodium chloride

2D representations of ionic compounds

A 2D representation is the easiest model to draw.

Sodium and chloride ions tightly packed in rows and columns.A two-dimensional model for the ionic lattice in sodium chloride

A 2D model shows the arrangement of one layer of ions but it does have a major additional limitation - it does not show where the ions are located on the other layers. This is important because there are different possible arrangements of ions.

Question

Use the 3D model and diagrams to help you draw the next row of sodium chloride in a 2D model.

In sodium chloride the ions are arranged alternately so, in the next row, chloride ions are located above the sodium ions.