This reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.
Add 2 molecules of hydrogen peroxide and 2 molecules of water. Since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.
Molecules are made up of atoms and are the smallest parts of compounds that still have properties. Atoms are the smallest parts of elements that still have properties.
