As we have already mentioned, a number of changes can occur when elements react with one another. These changes may either be physical or chemical. One way of representing these changes is through balanced chemical equations. A chemical equation describes a chemical reaction by using symbols for the elements involved. For example, if we look at the reaction between iron (FeFe) and sulphur (SS) to form iron sulphide (FeSFeS), we could represent these changes either in words or using chemical symbols:

iron+sulphur→iron sulphideiron+sulphur→iron sulphide

or

Fe+S→FeSFe+S→FeS

Another example would be:

ammonia+oxygen→nitric oxide+waterammonia+oxygen→nitric oxide+water

or

4NH3+5O2 → 4NO+6H2O4NH3+5O2→4NO+6H2O

Compounds on the left of the arrow are called the reactants and these are needed for the reaction to take place. In this equation, the reactants are ammonia and oxygen. The compounds on the right are called the products and these are what is formed from the reaction.

In order to be able to write a balanced chemical equation, there are a number of important things that need to be done:

We will look at each of these steps separately in the next sections.

It is very important to know the chemical symbols for common elements in the Periodic Table, so that you are able to write chemical equations and to recognise different compounds.

A chemical formula is a concise way of giving information about the atoms that make up a particular chemical compound. A chemical formula shows each element by its symbol and also shows how many atoms of each element are found in that compound. The number of atoms (if greater than one) is shown as a subscript.

Examples:

CH4CH4 (methane)

Number of atoms: (1×carbon)+(4×hydrogen)=5(1×carbon)+(4×hydrogen)=5 atoms in one methane molecule

H2SO4H2SO4 (sulphuric acid)

Number of atoms: (2×hydrogen)+(1×sulphur)+(4×oxygen)=7(2×hydrogen)+(1×sulphur)+(4×oxygen)=7 atoms in one molecule of sulphuric acid

A chemical formula may also give information about how the atoms are arranged in a molecule if it is written in a particular way. A molecule of ethane, for example, has the chemical formula C2H6C2H6. This formula tells us how many atoms of each element are in the molecule, but doesn't tell us anything about how these atoms are arranged. In fact, each carbon atom in the ethane molecule is bonded to three hydrogen atoms. Another way of writing the formula for ethane is CH3CH3CH3CH3. The number of atoms of each element has not changed, but this formula gives us more information about how the atoms are arranged in relation to each other.

The slightly tricky part of writing chemical formulae comes when you have to work out the ratio in which the elements combine. For example, you may know that sodium (NaNa) and chlorine (ClCl) react to form sodium chloride, but how do you know that in each molecule of sodium chloride there is only one atom of sodium for every one atom of chlorine? It all comes down to the valency of an atom or group of atoms. Valency is the number of bonds that an element can form with another element. Working out the chemical formulae of chemical compounds using their valency, will be covered in Grade 11. For now, we will use formulae that you already know.