Why equations must balance
Look carefully at the equation Mg + O2 → MgO. On the left there are two oxygen atoms; on the right there is one. That cannot be.
The law of conservation of mass
In a reaction no atoms are destroyed and none are created. So the number of atoms of each element in the reactants must equal the number in the products.
Balancing
Making the number of atoms in the reactants equal to the number in the products is called balancing the equation.
What do we balance with?
We balance by putting coefficients in front of formulae — never by changing a formula itself.
Right — adding a coefficient
2MgO
This means two molecules of MgO. The substance is still magnesium oxide.
Wrong — changing the formula
MgO2
This is an entirely different substance — and it is not what this reaction produces.
This matters greatly
We cannot write MgO2 to even up the oxygen. The reason is that the formula of the magnesium oxide obtained from this reaction is MgO. The reaction fixes the formula; all we may alter is the number in front of it.
Remember
Never change a formula. An equation is balanced only by placing coefficients in front of the formulae.