AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Lone pair–lone pair repulsion is greater than lone pair– bond pair…
Full specification wording
Shapes of simple molecules and ions: Lone pair–lone pair repulsion is greater than lone pair– bond pair repulsion, which is greater than bond pair–bond pair repulsion.
Not all repulsions are equal.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 3 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Not all repulsions are equal. Lone pair to lone pair repulsion is the strongest, lone pair to bonding pair is intermediate, and bonding pair to bonding pair is the weakest, because a lone pair is held by one nucleus only and so spreads closer to the central atom. The consequence is that each lone pair squeezes the remaining bond angles by roughly 2.5°. Methane, with four bonding pairs, keeps the full 109.5°. Ammonia, with three bonding pairs and one lone pair, closes to 107°. Water, with two bonding pairs and two lone pairs, closes further to 104.5°. The same reasoning gives about 104° for a four-pair ion carrying two lone pairs, such as ClF₂⁺.
What earns the marks
- The order lone pair to lone pair greater than lone pair to bonding pair greater than bonding pair to bonding pair.
- A lone pair occupying more space around the central atom than a bonding pair.
- The number of lone pairs on the central atom correctly identified.
- A bond angle consistent with that number of lone pairs.
Common mistakes and how to put them right
Reversing the order so bonding pairs are said to repel most strongly.
Saying lone pairs push bonding pairs further apart, widening the angle.
Swapping the angles, giving water 107° and ammonia 104.5°.
Reducing the angle by 2.5° once regardless of how many lone pairs are present.
Exam tips
- Write the repulsion order out in full using the words greater than; a shorthand ordering can be misread.
- Subtract 2.5° per lone pair from the base angle of the arrangement and state that is what you have done.
- Angles within a range are usually accepted, so a reasoned value close to the accepted one still scores.
Try the lesson
The start of the Shapes of simple molecules and ions: Lone pair–lone pair repulsion is greater than lone pair– bond pair repulsion, which is greater than bond pair–bond pair repulsion. lesson, made from checked teaching notes for this specification point.
Have a go first
Which sequence correctly lists the types of electron pair repulsion in order of decreasing strength (strongest first)?
Listen
Not all repulsions are equal. Lone pair to lone pair repulsion is the strongest, lone pair to bonding pair is intermediate, and bonding pair to bonding pair is the weakest, because a lone pair is held by one nucleus only and so spreads closer to the central atom. The consequence is that each lone pair squeezes the remaining bond angles by roughly 2.5°. Methane, with four bonding pairs, keeps the full 109.5°. Ammonia, with three bonding pairs and one lone pair, closes to 107°. Water, with two bonding pairs and two lone pairs, closes further to 104.5°. The same reasoning gives about 104° for a four-pair ion carrying two lone pairs, such as ClF₂⁺. A strong answer earns marks for these points. the order lone pair to lone pair greater than lone pair to bonding pair greater than bonding pair to bonding pair. a lone pair occupying more space around the central atom than a bonding pair. the number of lone pairs on the central atom correctly identified. a bond angle consistent with that number of lone pairs. Exam tip: Write the repulsion order out in full using the words greater than; a shorthand ordering can be misread.
Key words
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order of repulsion strengths
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lone pairs closing bond angles
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methane, ammonia and water
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angles in ions
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See every definition
- order of repulsion strengths
- Lone pair–lone pair repulsion is greater than lone pair–bond pair repulsion, which is greater than bond pair–bond pair repulsion.
- lone pairs closing bond angles
- Each lone pair squeezes the remaining bond angles by roughly 2.5°, because a lone pair is held by one nucleus only and so spreads closer to the central atom.
- methane, ammonia and water
- Methane keeps the full 109.5°, ammonia closes to 107° and water closes further to 104.5°.
- angles in ions
- The same reasoning gives about 104° for a four-pair ion carrying two lone pairs.
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