A chemical equation represents an actual chemical reaction.
A chemical reaction is a chemical change, a process during which elements and/or compounds are reacted together.
In simple terms a chemical equation can be expressed as a proceess during which substances that are about to undergo a chemical change, called reactants become products of that chemical change with either the addition or the release of a certain amount of energy.
i.e. Reactants -------> Products
Chemical equations can be written as word equations. For example the combination reaction between magnesium and oxygen can be written as
Magnesium(s) + Oxygen(g) --> Magnesium Oxide(s) + energy
Note that in the above expression the state of matter of the reactants (Magnesium - solid and Oxygen - gas), and of the products (Magnesium Oxyde - solid) is indicated as a subscript after each reactant and product. The expression also tells us that the reaction released energy. This is what we call in Chemistry an exothermic reaction.
The above reaction can be also expressed using chemical symbols. This is generally the preferred practice when we write chemical reactions.
Note that Oxygen gas, O2 (g) is diatomic. This means that a molecule of Oxygen gas consists of two oxygen atoms joined together. We will discuss this when we will study Bonding.
Mg (s) + O2 (g) ---> MgO(s)
Chemical equations observe the Law of Conservation of Mass which states that during a chemical reaction (or chemical change), in a closed system, the total mass of the reactants before the reaction must equal the total mass of the products after the reaction.
Therefore chemical equations must always be balanced. The above equation is not currently balanced as written because on the side of the reactants we have more atoms than on the side of the products.
We have in fact 1 atom of magnesium and two atoms of oxygen (i.e. 3 atoms of reactants) and only 2 atoms of the product magnesium oxyde.
Shown below is the balanced chemical equation for the reaction between magnesium and oxygen:
2 Mg (s) + O2 (g) ---> 2 MgO(s)
Now in the above equation, there are 4 atoms on the left side of the equation (2 Mg + 2 O) and 4 atoms on the right side of the equation (2 Mg + 2 O) and therefore the equation is balanced.