The oxidation number or oxidation state of an atom in a molecule or a polyatomic ion is the charge it would bear if all its bonds to unlike atoms were ionic bonds.
eg. 1: Methane
There are no bonds connecting like atoms in the methane molecule.
electronegativity: C > H
eg. 2: Acetaldehyde
There is one bond connecting like atoms in the acetaldehyde molecule, the carbon-carbon bond.
electronegativity: O > C > H
eg. 3: Nitric acid
There are no bonds connecting like atoms in the nitric acid molecule.
electronegativity: O > N > H
eg. 4: Periodate ion
There are no bonds connecting like atoms in the periodate ion.
electronegativity: O > I
