A compound is a substance that contains two or more different elements joined together. Each compound has a name and also a unique chemical formula. The formula can be made up of symbols and numbers. The chemical formula for methane, shown above, is CH4.
The chemical formula tells us what elements make up the compound, and also how many atoms of each element are in that compound.
If there are numbers in a compound's formula, the numbers will be written smaller, and are placed slightly lower down than the symbols.
For example, if we take a look at the compound above with the chemical formula H2O (do you recognise it - it's water!) we can tell quite a lot about the compound.
We can tell that it is made up of hydrogen (H) and oxygen (O). And we can also tell that there are two atoms of hydrogen for every atom of oxygen - that's what the little 2 means after the H.
So for methane, which we saw at the start, the chemical formula is CH4. This means it is made up of two elements - carbon and hydrogen. There is one atom of carbon in methane, and four atoms of hydrogen.
Let's have a look at some common compounds and try some questions about what elements they contain.
It might be helpful to have a paper copy of the periodic table in front of you for these questions.