The effect of electron pair repulsion on bond angles.

The idea

Bond angle follows from two numbers: the total electron pairs, which sets the base arrangement, and how many of those are lone pairs. With no lone pairs the base angle stands: methane and the ammonium ion are 109.5°, boron trifluoride 120°, carbon dioxide 180°, sulfur hexafluoride 90°. For a tetrahedral base, each lone pair closes the angle by about 2.5°: ammonia is 107°, water 104.5°. This 2.5° rule does not apply to all geometries; for example, square planar shapes maintain 90° angles as lone pair repulsions cancel. A trigonal bipyramid has two different angles, 120° equatorial and 90° axial. Quote an angle consistent with the shape you drew to avoid contradicting yourself.

What earns the marks

• The total number of electron pairs around the central atom.
• The number of those pairs that are lone pairs.
• The base bond angle of that arrangement.
• The final bond angle stated for the shape to the nearest 0.5°.

Common mistakes and how to put them right

• Forgetting to adjust the electron count for the charge on an ion, so the lone pair count is wrong.

• Quoting the unchanged base angle when lone pairs are present.

• Applying the 2.5° reduction rule to non-tetrahedral geometries like square planar or octahedral.

• Quoting an angle that contradicts the shape drawn alongside it.

Exam tips

• Write the pair count and lone pair count beside the diagram so the examiner can follow your reasoning.
• For trigonal bipyramidal species give both angles and say which positions each refers to.
• Use the 2.5° reduction rule specifically for tetrahedral base shapes (109.5° to 107° to 104.5°), but remember it does not apply universally to all geometries.