This KS3 Science quiz takes a look at reactions, equations and formulae. When atoms of two or more elements join together, a chemical reaction has taken place. It is also possible to get elements and compounds to react with each other too, for example a metal and an acid. In that case, the atoms are rearranged during the reaction and new substances are formed. When you are writing down a chemical equation the substances on the left hand side are what you started with. Those on the right hand side will be different and they are what you end up with. We call the starting materials reactants (reagents means the same) and the finishing materials are the products.
Some people find writing chemical formulae a mystery. Others find it easy. A formula is a short way of writing down what atoms are present in an element or compound.
Writing down the symbol of an element means there is one atom of the element there. Writing down the symbol of an element with a small number after it tells you how many atoms of the element there are. So writing down the symbol Fe means 'here is one atom of the element iron' and O2 means 'here we have two atoms of the element oxygen chemically joined together'. Which would you prefer to write down - the longhand or the shorthand?