L.YADAVPGT (CHEMISTRY)

KV C.O.D JABALPUR

FRACTIONAL OXIDATION

NUMBER

  Valency Oxidation number1 Valency of an element is the number of

hydrogen atoms or twice the number of oxygen atoms which combine with one atom of the element.

Oxidation number is the charge assigned to an atom of a molecule or ion according to some arbitrary rules.

2 Valency of an element is only a number without any sign. For example, valency of hydrogen is 1 and that of oxygen is 2.

The oxidation number of an element has either positive or negative sign. For example the oxidation number of hydrogen is +1 while that of oxygen is - 2in its compounds.

3 Except for noble gases , valency of no other element is zero.

Oxidation number of an element can be zero. For example, in C6H12 O6 , the oxidation number of carbon is zero.

4 Valency of an element is always a whole number .

The oxidation number of element can be even fractional . For example in Na2S4 O6 , the O.N of sulphur is 2.5.

5 In most of the cases , an element has fixed valency in all its compounds. For example, carbon is tetravalent.

The oxidation number of an element is usually different compounds. For example, carbon atom has oxidation number −4 (CH 4 ) , − 3(C2H6 ) , +2 (CHCl3) and 0 (C5 H12O6 ).

DETERMINATION OF OXIDATION NUMBER

(a) Oxidation number of an element IN ITS PURE STATE is zero. For example- H2 , S8, P4 , O3

(b) IN COMBINED STATE-(1)Oxidation number of ‘F’ is always -1.(2)Oxidation number of ‘O’ is -2. EXCEPTION- in peroxides it is -1 and in super oxides it is -1/2(3)Oxidation number of alkali metal is always +1(4) oxidation number of ‘H’ is -1 in metal hydrides and +1 in

non-metal hydrides.(5)Oxidation number of alkaline metal is always +2

(c) Total oxidation number of a molecule is zero and total oxidation number of an ion is equal to its charge.

(d) Maximum oxidation number of an element is equal to its group number.

FRACTIONAL OXIDATION NUMBERIt is the average oxidation state of an element when two or more of its atoms are present in different oxidation states in a given compound.It is determine with the help of bond structure method.RULES FOR DETERMING THE OXIDATION NUMBER BY BOND STRUCTURE METHOD(1)For a single covalent bond oxidation number is 1 unit i.e.

For the most electropositive element it is +1 and for the more electronegative element it is -1

(2) for a double covalent bond oxidation number is 2 unit i.e for the more electronegative element it is -2 and for the more electropositive element it is +2

(3)For a triple covalent bond oxidation number is unit 3 i.e. for the more electropositive element it is +3 and for the most electronegative element it is -3.

(4) for a co-ordinate bond oxidation number for more electronegative element is -2 and for the more electropositive element it is +2. but if the donor atom is more electronegative then the co-ordinate bond is not determined for the oxidation number.

SOME EXAMPLES OF COMPOUNDS SHOWING FRACTIONAL OXIDATION NUMBERS1.OXIDATION NUMBER OF CARBON IN CARBON SUBOXIDE

AVERAGE OXIDATION NUMBER=2+0+2/3=4/3

+2 0 +2

AVERAGE OXIDATION NUMBER=6+4+6/3=16/3

2.OXIDATION NUMBER OF BROMINE IN

TRIBROMOCTAOXIDE

Br Br

Br O

O

O

O

O

O O

O

3. OXIDATION NUMBER OF SULPHUR IN SULPHUR TETRA THIONATE ION

+5

0

0

+5

AVERAGE OXIDATION NUMBER=5+0+0+5/4=10/4 OR 5/2

0

0

4.FERROUS IN FERROUS FERRIC OXIDE(MAGNETITE )

AVERAGE OXIDATION NUMBER=2+3+3/3=8/3

2+2

2+32+3

+4+2

+2

5.OXIDATION NUMBER OF LEAD(Pb) IN Pb3O4

AVERAGE OXIDATION NUMBER=4+2+2/3=8/3

DETERMINE THE OXIDATION NUMBER OF THE UNDERLINED

ELEMENTS IN GIVEN COMPOUNDS

EXERCISE

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