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Chemical formulae
The chemical formula of a compound tells you how many atoms of each element the molecule contains. .
Formulae of some compounds, with the number of atoms of each element in the molecule
| carbon monoxide | carbon dioxide | sulfur dioxide | nitrogen monoxide | nitrogen dioxide | water | |
|---|---|---|---|---|---|---|
| formula | CO | CO2 | SO2 | NO | NO2 | H2O |
| carbon atoms | 1 | 1 | ||||
| sulfur atoms | 1 | |||||
| nitrogen atoms | 1 | 1 | ||||
| hydrogen atoms | 2 | |||||
| oxygen atoms | 1 | 2 | 2 | 1 | 2 | 1 |
The small 2 after an element tells you there are two atoms of that particular element in each molecule. For example, the water molecule H2O has two hydrogen atoms. Notice that you do not write a number 1 if there is only one atom of an element in a molecule.
Brackets
Some formulae have brackets in them. For example, sodium hydroxide is NaOH, but magnesium hydroxide is Mg(OH)2. The 2 outside the brackets tells you that you have two of each atom inside the bracket. So in Mg(OH)2 you have one magnesium atom, two oxygen atoms and two hydrogen atoms.
Examples of formulae with brackets
| formula | iron atoms | oxygen atoms | hydrogen atoms | nitrogen atoms | sulfur atoms | total number of atoms |
|---|---|---|---|---|---|---|
| Fe(OH)3 | 1 | 3 | 3 | 7 | ||
| Fe(NO3)3 | 1 | 3 × 3 = 9 | 3 × 1 = 3 | 13 | ||
| (NH4)2SO4 | 4 | 2 × 4 = 8 | 2 × 1 = 2 | 1 | 15 |
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