The concept of oxidation number and the definitions of oxidant and reductant are critical to an understanding of redox chemistry. In redox reactions
 

• a species known as an oxidant accepts electrons.
  As a consequence of reaction, the oxidation number of an atom in the oxidant becomes more negative.


• a species known as a reductant donates electrons.
  As a consequence of reaction, their oxidation number becomes more positive.

The oxidation number is

• 0 if the substance is an element.


• equal to the charge on a monatomic ion


• +1 for hydrogen in its compounds with nonmetals


• -2 for oxygen except in compounds having O-O bonds where it is -1


• –1 for halogens except in species where the halogen is bonded to oxygen

Oxidation numbers of other bonded atoms can be determined becausethe sum of oxidation numbers on the atoms in a
 

• compound equals zero


• polyatomic ion equals the overall charge.