(a) Oxidation number
The oxidation number is the formal electrical charge of a component atom in a
compound or in an ion allocated in such a way that the atom with smaller
electronegativity has a positive charge. Since electrical charges do not differ in the case of
a molecule composed of the same atoms, the oxidation number of the atoms is the
quotient of the net electrical charge divided by the number of atoms. In the case of a
compound or an ion consisting of different atoms, the atoms with larger electronegativity
can be considered as anions and those with smaller electronegativity as cations. For
example, nitrogen is 0 valent in N2; oxygen is -1 in O2
2-; nitrogen is +4 and oxygen -2 in
NO2; but nitrogen is -3 and hydrogen +1 in NH3. That is, the oxidation number can be
different for the same atom combined with different partners and the atom is said to be in
the formal oxidation state corresponding to that oxidation number. Although this does not
express the quantitative deviation of the actual electric charge, it is convenient in
counting valence electrons or in dealing with redox reactions.