How to find oxidation number of atom using oxidation number rules?

If you want to find the oxidation  number of an atom either it is in its elemental form, in the form of  monoatomic ion and its in a compound, in all circumstances you have to use the oxidation number rule.Click on this link to check the table of oxidation number rules and learn how to apply them in the real problems below.

Sample Problem 1 : What is the oxidation number of Fe in Fe3+ ?

Solution : According to the rule 2 the oxidation number of a monoatomic ion equals the charge of that ion,so the oxidation number of Fe in  Fe3+ is  3.

Sample Problem 2 : Find the oxidation number of chlorine atom in each of the following (a) HClO4 (b) ClO3-

Solution : According to rule 4 : The sum of oxidation numbers of atoms in a polyatomic ion equals to the charge of that ion.So, the oxidation number of O is -2 according to the rule 6 and assume the oxidation number of  Cl as x, so as per rule 4

in CLO3- 

x + (-2) * 3 = -1

x -6 = -1

x = -1 +6

x = +5

oxidation number of Cl in CLO3- = +5

According to rule 3 : The sum of oxidation numbers of atom in a compound is zero as per rule 3 and  H is +1 as per rule 10 ,hence

in HClO4

+1 + x + (-2)*4 = 0

+1 + x -6=0

x = +5

oxidation number of Cl is +5 in HClO4

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