A Level - Exam Technique: Shapes of Molecules

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:

Source

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!