.....when all the bonding pairs of electrons are removed, with the charge assigned to central atom:
#1.# #"The oxidation number of a free element is always 0."#
#2.# #"The oxidation number of a mono-atomic ion is equal"# #"to the charge of the ion."#
#3.# #"For a given bond, X-Y, the bond is split to give "X^+# #"and"# #Y^-#, #"where Y is more electronegative than X."#
#4.# #"The oxidation number of H is +I, but it is -I in when"# #"combined with less electronegative elements."#
#5.# #"The oxidation number of O in its"# compounds #"is usually -II, but it is -I in peroxides."#
#6.# #"The oxidation number of a Group 1 element"# #"in a compound is +I."#
#7.# #"The oxidation number of a Group 2 element in a"# #"compound is +II."#
#8.# #"The oxidation number of a Group 17 element"# #"in a binary compound is -I."#
#9.# #"The sum of the oxidation numbers of all of the atoms"# #"in a neutral compound is 0."#
#10.# #"The sum of the oxidation numbers in a polyatomic"# #"ion is equal to the charge of the ion."#
The given rules are a formalism. When we take an iron salt, or an iron complex, i.e. #FeSO_4#, or #FeCl_2#, or #Fe(OH_2)_6^(2+)#, the charge on the IRON ATOM is formally #II+#; i.e. it is a #"ferrous ion"#. In #Fe_2(SO_4)_3#, or #FeCl_3#, or #Fe(OH_2)_6^(3+)#, we has #Fe^(3+)#, #"the ferric ion"#, #Fe(III+)#. Iron complexes with different oxidation states are more rare.
