Disproportionation happens when an electron from one atom is taken by another one, making two different chemicals. It is the opposite of symproportionation, which is the giving of electrons from one atom to another atom, It makes one chemical from two different chemicals.
Example of disproportionation
Sodium hypochlorite (household bleach) reacts with itself when it is dried. One atom gives two electrons away to two other atoms. The first atom becomes sodium chlorate, while the other two that got the electrons becomes sodium chloride. 3 NaClO → 2 NaCl + NaClO3
Example of symproportionation
Ferric ions react with iron metal. Two ferric ions give two electrons (one electron for each atom) to one iron atom. The ferric ion becomes a ferrous ion. The iron also becomes a ferrous ion. 2 Fe3+ + Fe → 3 Fe2+