Shapes of molecules can be difficult, but there are several key criteria to hit in an exam question that will help you maximise your marks:
- number of bonding pairs
- number of lone pairs
- electron pairs repel
- strength of bond pair - bond pair repulsion vs. bond pair - lone pair repulsion
- bond angle and/or shape
For example, consider this question about ammonia:
Q11. a) State and explain the bond angle and shape of an ammonia molecule
(4 marks)
In the following answer, I have broken each marking point down by alternating colours, and each relates to the points listed above.
Ammonia contains 3 bonding pair and 1 lone pair. The electron pairs repel, but the bond pair - bond pair repulsion is weaker than the bond pair - lone pair repulsion, therefore constricting the bond angle to 107° and making the shape pyramidal.
So, if you follow the logic of addressing those 5 points above then you won't go far wrong in an exam situation - good luck!
