This follows on nicely from the last section because it is based around valencies. For this section you don’t need to understand where valency comes from you just need to know how to read tables. You will need to refer to the valency table and periodic table:

The steps you need to take in writing a chemical equation are thus:

1.      Write out the symbols of the elements (for example sodium chloride = NaCl)

2.      Write the valencies of the elements below their symbol

3.      Swap the valencies around

4.      Take the number from 3 and use it to denote the number of atoms of that substance (i.e if there is a two under hydrogen it gives H₂)

5.      Simplify if possible

Let’s go through an example following the steps above:

Write the chemical formulae for water (hydrogen and oxygen)

Symbols:                       H  O

Valencies                       1  2

Swap around:                  2   1

Write Formulae:               H₂O     

Now, we’re going to do a more annoying example:

Write the chemical formulae for Aluminium Nitride 

We just go through the same steps except this one we have to simplify.

Al  N

3  3

3  3 

Al₃N₃

This can be simplified to AlN because both divide by the same number to give a whole number – in this case three.