Covalent bonding: describe the limitations of using dot and cross, ball and stick, two and three-dimensional diagrams to represent molecules or giant structures

The idea

Every model of a molecule or giant structure is a simplification, and each type has particular limitations. Dot and cross diagrams show which electrons are shared and can show lone pairs, but they do not show the shape of the molecule, the relative sizes of atoms or the three-dimensional arrangement. Ball and stick models show the shape and the angles between bonds clearly, but the balls are not the real size of the atoms and the sticks suggest a solid connection where there is actually empty space. Two-dimensional diagrams are quick to draw and show bonding, but they flatten a three-dimensional structure and can hide which atoms are in front. Three-dimensional models show arrangement in space, but they can still misrepresent bond lengths, electron clouds and the scale of the structure.

What earns the marks

• Dot and cross diagrams show shared and lone pairs of electrons but do not show molecular shape or the relative sizes of atoms.
• Ball and stick models show bond angles and three-dimensional shape but the balls are not to scale and the sticks wrongly suggest a physical rod between atoms.
• Two-dimensional diagrams are easy to draw and show connectivity but lose the three-dimensional arrangement of atoms.
• Three-dimensional models show spatial arrangement but may still misrepresent bond lengths, electron density and the true scale of a giant structure.
• A good answer names the model, states one specific limitation and explains why that limitation matters for understanding the structure.

Common mistakes and how to put them right

• Saying dot and cross diagrams are wrong.

  Put it right: The error is confusing a limitation with an error, so the correction is to say they are useful but do not show shape or relative size.

• Claiming ball and stick models show the real size of atoms.

  Put it right: The error is that the balls are only representative, so the correction is to state that they are not to scale.

• Describing a limitation without linking it to the model.

  Put it right: The error is a vague statement such as 'it is not accurate', so the correction is to name the model and say exactly what it fails to show.

Exam tips

• Use the structure 'model X is useful because ..., but it does not show ...' to make the limitation clear.
• Give a different limitation for each model rather than repeating the same point.
• Link each limitation to a consequence, such as not being able to predict the shape or the reactivity of a molecule.