Describe the limitations of using dot and cross, ball and stick, two and…

The idea

Every model of a giant ionic structure is a simplification, so each has limitations. Dot and cross diagrams show electron transfer and charges clearly, but they usually show only a few ions, so they hide the huge repeating lattice and give no sense of scale or three-dimensional shape. Ball and stick models show the arrangement and relative positions of ions, but the sticks suggest directional bonds, which ionic lattices do not have, and the balls are far too small compared with the spaces between ions. Two-dimensional diagrams flatten a three-dimensional lattice, so they miss the way ions surround each other in all directions. Three-dimensional models are better but still cannot show the true scale, the strong electrostatic forces in every direction, or the fact that the lattice is effectively endless.

What earns the marks

• Explain that dot and cross diagrams show only a small number of ions, so they do not convey the giant repeating nature of the lattice.
• State that ball and stick models use sticks that imply directional covalent bonds, whereas ionic attraction acts in all directions.
• Describe how two-dimensional diagrams flatten a three-dimensional lattice and so misrepresent the arrangement of ions.
• Note that models do not show the true scale, because ions and the distances between them are not drawn to scale.
• Recognise that even three-dimensional models cannot show the lattice as effectively infinite or fully represent the electrostatic forces.

Common mistakes and how to put them right

• Saying ball and stick models show the ions touching.

  Put it right: The balls are much smaller than the spaces between ions, so the model is not to scale.

• Claiming dot and cross diagrams show the whole lattice.

  Put it right: They show only a few ions and omit the repeating three-dimensional pattern.

• Stating that the sticks in ball and stick models represent ionic bonds in fixed directions.

  Put it right: Ionic attraction is non-directional and acts in all directions.

Exam tips

• Name the specific model in each limitation, so the examiner can see which diagram you are criticising.
• Contrast what the model does show with what it fails to show, for example charges shown but scale missing.
• Use precise terms such as lattice, three-dimensional, scale and electrostatic forces rather than vague words like 'not accurate'.