How can I tell if a chemical reaction is balanced or not balanced?

If the equation has the same number of each type of atom on both sides of the arrow, it is balanced. If the number of atoms on each side of the arrow is not the same, the equation is not balanced. For example, in the equation H2 + O2 -->H2O, there are two hydrogen atoms and two oxygen atoms on the left, and two hydrogen atoms and one oxygen atom on the right. Therefore, the equation is not balanced. However, in the equation 2H2 + O2 -->2H2O, there are four hydrogen atoms and two oxygen atoms on the left and four hydrogen atoms and two oxygen atoms on the right, so the equation is balanced.