When writing the chemical equation, it is advisable to follow three steps :
(a) Write down the equation in words.
(b) Then, write down the correct chemical formula for each reactant and product.
(c) Balance the equation. Make sure that the number of atoms of each substance before and after the reaction is the same.
(d) Put in the state symbols of every reactant and product. Solid is (s), liquid is (1), gas is (g) and aqueous is (aq). Aqueous means dissolved in water.
For example, take the equation that represents the combustion of magnesium in oxygen.