What is a Chemical Formula?
When atoms join together to form compounds, chemists use a shorthand notation called a chemical formula to represent exactly which atoms are present and how many of each.
A chemical formula is a shorthand way of representing a chemical compound using the chemical symbols of its elements and small numbers (subscripts) to show how many atoms of each element are in one unit of the compound.
For example, water has the chemical formula H₂O. This tells us:
- There are 2 hydrogen (H) atoms
- There is 1 oxygen (O) atom
When no subscript number is written after a symbol, it means there is just 1 atom of that element. You never write H₁O — you simply write H₂O.
Think of a chemical formula like a recipe. H₂O is like saying "take 2 parts hydrogen and 1 part oxygen" — it tells you the exact ingredients and their amounts in every single molecule.
Reading Chemical Formulae
Let's look at some examples of chemical formulae and practise reading them.
Here are some compounds and what their formulae tell us:
| Compound | Formula | Atoms present |
|---|---|---|
| Water | H₂O | 2 H, 1 O |
| Iron sulfide | FeS | 1 Fe, 1 S |
| Carbon dioxide | CO₂ | 1 C, 2 O |
| Magnesium oxide | MgO | 1 Mg, 1 O |
| Sodium oxide | Na₂O | 2 Na, 1 O |
| Calcium carbonate | CaCO₃ | 1 Ca, 1 C, 3 O |
Notice that all the examples above are compounds — they contain atoms of two or more different elements bonded together. Pure oxygen gas (O₂) and ozone (O₃) are not compounds because they contain only one element. O₂ and O₃ are called allotropes of oxygen — different forms of the same element. Their formulae tell you how many atoms are bonded together, but they are not compounds.
Do not confuse the large numbers in front of a formula with the small subscript numbers inside it. In 2MgO, the large 2 means two separate MgO units, while the subscript in CO₂ means two oxygen atoms bonded inside one molecule.
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