FRACTIONAL
OXIDATION
NUMBER
Valency
Oxidation number
1
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 ).
(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.
It 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 NUMBERS
1.OXIDATION NUMBER OF CARBON IN CARBON
SUBOXIDE
+2
0
+2
AVERAGE OXIDATION NUMBER=2+0+2/3=4/3
O
O
O
Br
Br
O
O
O
Br
O
O
AVERAGE OXIDATION NUMBER=6+4+6/3=16/3
3. OXIDATION NUMBER OF SULPHUR IN
SULPHUR TETRA THIONATE ION
0
+5
0
+5
00
AVERAGE OXIDATION NUMBER=5+0+0+5/4=10/4 OR 5/2
4.FERROUS IN FERROUS FERRIC
OXIDE(MAGNETITE )
2+3
2+3
2+2
AVERAGE OXIDATION
NUMBER=2+3+3/3=8/3
5.OXIDATION NUMBER OF LEAD(Pb) IN
Pb3O4
+2
+4
+2
AVERAGE OXIDATION NUMBER=4+2+2/3=8/3
EXERCISE
DETERMINE THE OXIDATION NUMBER OF THE
UNDERLINED
ELEMENTS IN GIVEN COMPOUNDS
1.
2.
3.
4.
5.
6.
7.