An oxidation number is the charge of the ion that would form if a covalent molecule was ionic. Oxidation numbers exist for covalent compounds, as ions are not formed.
The oxidation number is determined by the respective electronegativity of the atoms involved. A more electronegative element will always have a negative oxidation number. In the compound, all the oxidation numbers must add up to 0. Oxidation numbers of elements can vary between compunds.
A few rules to follow about oxidation numbers
Fluorine will always be -1
Oxygen will be -2 unless in a compound with fluorine
Hydrogen is usually +1
A few examples
HF (Hydrogen flouride) Oxy number of H= +1 and F = -1
H2O Oxy number of H = +1 and O= -2
NH3 Oxy of H= +1 and since there are 3 Hydrogens each with an oxidation number of +1 N must have an oxy number of -3 for the overall oxidation number of the compound to be 0.
H2SO4 Oxy number of H= +1 O=-2 When adding up the hydrogen and oxygen oxidation numbers ( 1 + 1 -2 - 2 -2 -2) we are left with -6 thus S must have an oxidation number of +6 for the overall number to be 0.
NO2 Oxidation number of O= -2 there are 2 oxygens leaving us with -4 thus N must have an Oxy number of +4 for the overall number to be 0.
