BALANCING CHEMICAL EQUATION

Balancing the chemical equation means the number of atoms of each element involved in the reaction should be the same on the reactant side and product side
eg:                                           Mg + O₂−−>MgO

In this equation, magnesium has one atom in the reactant so in the product also there should be only one (already correct here)

Then check for oxygen, it contains 2 atoms in reactant so in the product also the number of oxygen should be 2. so correct it like as below

Mg + O₂−−>2MgO

But when it is corrected number of magnesium in the product and reactant change. so fix it like as below

2Mg + O₂−−>2MgO

Thus the equation is balanced 
ie: no: of magnesium and oxygen in the reactant is equal to no: of magnesium and oxygen in the product 

WHAT IS THE PURPOSE OF BALANCING A CHEMICAL EQUATION?

                   According to the law of conservation of mass:- mass can neither be created nor be destroyed in a chemical reaction. So the total mass of elements present in reactants of a chemical reaction has to be equal to the total mass of elements present in the product. In other words, the number of atoms of each element remains the same before and after the chemical reaction. Hence we need to balance a chemical equation