Formula is a system of writing a compound using the symbols for the elements they contain and subscript numbers show how many of each type of atom is bonded to another atom

For example H₂O show there are 2 hydrogen atoms bonded to 1 oxygen atoms

This power point explains how they are named

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BBC Bitesize explains the formula of compounds: www.bbc.co.uk/bitesize/guides/zt2hpv4/revision/4

Here BBC Bitesize links the particle diagrams of molecules to their formula: www.bbc.co.uk/bitesize/guides/zt2hpv4/revision/5