There are several chemical reactions in which oxidation reduction takes place.

`text(Oxidation is)`
i) the gain of oxygen
ii) the loss of hydrogen
iii) the loss of electrons
iv) the increase of O.N.

`text(Reduction is)`
i) the loss of oxygen
ii) the gain of hydrogen
iii) the gain of electron
iv) the decrease in O.N.

To describe these changes, the concept of oxidation state becomes necessary. For ionic species, the charge on each ion is said to be the oxidation state for that atom. For example in `NaCl`. Na exists as `Na^+` and `Cl` exists as `Cl^-`. Therefore the oxidation state of `Na` in `NaCI` is `+ 1` and that of `Cl^-` is `- 1`. But in covalent molecules, the charge on an atom would be so small that sometimes it becomes impossible to calculate the exact charge on each atom of a molecule. Therefore, the Oxidation State (O.S.) or Oxidation Number (O.N.) is defined as the charge, an atom would have in a molecule if all the bonds associated with this atom in the molecule are considered to be completely ionic. For example in `H_2O` there are two `O -H` bonds. If we assume both the `O- H` bonds to be completely ionic, then each `H ` would possess a charge of `+ 1`, while 0 possess a charge of `- 2`. This is because oxygen is more electronegative than hydrogen. On the other hand, in `H_2O_2` there are two `O-H` bonds and one `O - O` bond. Considering each `O-H` bond to be ionic both the oxygen atoms acquire a charge of `1` and both the `H`, `+ 1`. This is because `O -O` bond can not be assumed to be ionic as both the atoms have the same electronegativity.