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Question from Charlotte |
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Ask a Teacher - Chemistry - Formulae and Equations - A Question from Charlotte |
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| Student Name |
Charlotte |
| Question |
As part of the acids and alkalis test we have to write out fomulae. I am not very good at remembering how to do them - could you help me find a easier way of remembering?
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| Answer |
The only way to be able to work out formulae is by learning a method such as the valency method:
Valence numbers are used to work out the number of atoms of each element in a formula.
The valency of elements in groups 1-4 in the periodic table is the same as the group number. The valency of the elements in groups 5 to 7 is 8 minus the group number.
The valency of the transition metals is indicated by a Roman numeral in brackets after the metal.
Example: Work out the formula for iron(ii) chloride.
| Elements, |
group, |
valency, |
(swap valency), |
formula |
| Fe |
- |
2 |
1 |
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FeCl2
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| Cl |
7 |
8-7=1 |
2 |
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The name can also tell you the formula eg nitrogen dioxide – the di- tells you there are two oxygens, so the formula is NO2
You need lots of practice – have a look in your notes and try the examples there without looking at the answers. The Bitesize revision book has worked examples in the last chapter.
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