Oxidation Number – rules for calculation

oxidation number

source : compound chem.com

Oxidation number of an element in a particular compound is the no. of e’s lost or gained by the element during its change from free state to that compound.

General Rules for Assigning Oxidation Number-

1. In free state Oxidation number of an element is zero.
2. Oxi. no. can be zero or +ve or -ve
3. Oxi. no. of an ion is equal to the charge on that ion.

Ex. Na⁺=+1, Mg⁺⁺=+2, Al⁺⁺⁺=+3, O^{– –} =–2

4. The algebric sum of Oxi. no. of all the elements in a compound is equal to zero.

Ex. KMnO₄

Oxi. No. of K + Oxi No. of Mn +4xO.No. of Oxy= 0

5. In combined state, Oxi. no. of

F is always     –1

Oxy is –2, In peroxides is –1, In F₂O, Oxi. no. of

Oxy is –2

Oxi. no. of metal is always positive

Oxi. no. of alkali metal is always +1

Oxi. no. alkaline earthmetal is always +2

6. Oxi. no. of halogens (except F) is generally -1 but it may be +1,+3,+5,+7
7. The algebric sum of Oxidation number of elements in an ion is equal to the net charge on that ion.

Ex.      SO₄^{– –}

o.no. of S+ 4×0.No. of oxy= –2

8. Hydrogen has two possible oxidation numbers: +1 and -1Hydrogen has an oxi.no. of +1 when it is bonded to nonmetals, which are highlighted on the right side of the following periodic table.For the compound hydrochloric acid, hydrogen is bonded to chlorine, a nonmetal, so the oxi.no. of hydrogen is +1.
   

   Hydrogen has an oxi.no.of -1 when it is bonded to a metal.

   For the compound sodium hydride, hydrogen is bonded to sodium, which is a metal, so the oxidation number of hydrogen is -1.

9. Variable Oxidation number is mostly shown by transition elements & p-block elements.
10. Oxi. no. of Sn    = +2, +4

Oxi. no. of As

Oxi. no. of Fe    = +2, +3

Oxi. no. of Cu   = +1,+2

Oxi. no. of Mn  = +1,+2,+3,+4,+5,+6,+7