Remember that we use chemical symbols to stand for the elements. For example, C stands for carbon, O stands for oxygen, S stands for sulfur and Na stands for sodium. For a molecule, we use the chemical symbols of the atoms it contains to write down its formula. For example, the formula for carbon monoxide is CO. It tells you that each molecule of carbon monoxide consists of one carbon atom joined to one oxygen atom.
Take care when writing your symbols and formulae. Be careful about when to use capital letters. For example, CO means a molecule of carbon monoxide but Co is the symbol for cobalt (an element).
The word 'formulae' is the plural of 'formula'. If we have more than one formula, we don't say formulas, we say formulae.
We use numbers to show when a molecule contains more than one atom of an element. The numbers are written below the element symbol. For example, CO2 is the formula for carbon dioxide. It tells you that each molecule has one carbon atom and two oxygen atoms.
Take care when writing these formulae. The small numbers go at the bottom. For example CO2 is correct but CO2 is wrong.
Some formulae are more complicated. For example, the formula for sodium sulfate is Na2SO4. It tells you that sodium sulfate contains two sodium atoms (Na2), one sulfur atom (S) and four oxygen atoms (O4).
All compounds have a definite composition. For example, a water molecule always contains two hydrogen atoms and one oxygen atom. It cannot be a water molecule if it has different numbers of these atoms. The formula for water is always H2O.