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A compound is a substance that contains two or more elements that are chemically combined. The elements in a compound are present in fixed proportions. For example, carbon dioxide always has 12 g of carbon for every 32 g of oxygen.

A chemical formula can be used to represent a compound. The formula shows:

• the symbols for each element in the compound
• the number of atoms of each element in a unit of the compound

For example, magnesium oxide is made up of two elements, magnesium and oxygen. Its formula is MgO. This shows that it has one atom of magnesium for every one atom of oxygen.

Here are some more examples of compounds and their formulae. The subscript number in a formula shows if there is more than one atom of an element.

Many compounds exist naturally. They can also be formed from their elements in chemical reactions. In a chemical reaction, one or more new substances are formed. Most chemical reactions involve energy changes.

It is not easy to split up a compound into its elements - the only way to do this is in chemical reactions.

Chemical formulae of ions

An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons. The number and sign of its electrical charges are shown in superscript text.

Names and formulae of some common ions:

Simple formulae

The formula of an ionic compound can be predicted using the formulae of its ions. The numbers of ions in a formula must give an equal number of positive and negative charges.