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:

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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!

GCSE - Determining Ionic Formulae

Ionic compounds have a set formula, based on the charges of the ions they are made from. For example, calcium chloride is CaCl<sub>2

This is the case because the calcium ion has a charge of 2+ and each fo the chloride ions have a charge of 1-. Overall, this means that the two positive charges and the two negative charges cancel each other out.</sub> 

A good way of laying this out on paper is by using an ionic equation to show the formation of the ionic compound from its ions:

Ca²⁺ + Cl^- --> ?

So if we left the above equation as it is, we would get CaCl⁺ as only one of the positive charges on the calcium ion is cancelled by the negative charge on the chloride. 

However, if we were to introduce a second chloride ion in the formula, then the negative charge it has would cancel the second negative charge on the calcium ion:

Ca²⁺ + 2Cl^- --> CaCl₂

_{And that's it - that's why ionic substances have the formula that they do. I would strongly recommend that you get familiar with the list of ions below, as in the new GCSE's you don't get them given to you on a data sheet as previous years did.} 

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