The document defines oxidation number and provides rules for determining oxidation numbers of elements in compounds and polyatomic ions. The rules state that the oxidation number of atoms is 0, ions take the charge, and the sum of oxidation numbers in compounds and polyatomic ions equals the overall charge. Examples are provided to demonstrate applying the rules to calculate the oxidation number of underlined elements in various compounds and polyatomic ions.

1. Azlee Chemistry 2009
Oxidation number
(NOMBOR PENGOKSIDAAN)
Definition:
The oxidation number of an element is the
charge that the atom of the element would
have if complete transfer of electron occurs
Nombor pengoksidaan unsur adalah cas atom unsur itu
sendiri jika pemindahan elektron berlaku dengan
lengkap
Tips from the rules:
 the oxidation number for atom and molecule is
zero =0
Atom or molecule Zn C Mg Ag H2 O2 Cl2
Oxidation number 0 0 0 0 0 0 0
 the oxidation number for monoatomic ion is
equal to its charge
Monoatomic ion H+
Fe2+
Mn2+
Fe3+
Cr3-
O2-
N3-
Oxidation number +1 +2 +2 +3 -1 -2 -3

2. Azlee Chemistry 2009
 the sum of oxidation numbers of all elements in
the compound is zero
Compound H2SO4 KMnO4 SO2 Na2SO4 K2Cr2O7
Oxidation number
0 0 0 0 0
 the sum of oxidation number of all elements in
polyatomic ions is equal to the charge of the
ions
Polyatomic
ion
NH4
+
SO4
2-
MnO4
-
Cr2O7
2-
S2O3
2-
Fe(CN)6
3-
Oxidation
number
+1 -2 -1 -2 -2 -3

3. Azlee Chemistry 2009
Calculate the oxidation numbers for the underlined
elements.
(i) SO2 [compound]
[refer to rules above]
Element S O
Sum of oxidation
numbers
Number of element 1 2
0
Oxidation number x -2
[let the unknown oxidation number is equal to x]
1(x) + 2(-2) = 0
x - 4 = 0
x = +4
Thus;
Oxidation number of S in SO2 = +4
Latihan
Dapatkan nombor pengoksidaan bagi
 H2SO4
 Na2S2O3

4. Azlee Chemistry 2009
 NH3
 S2O3
2-

5. Azlee Chemistry 2009
(ii) H2SO4
Element H S O
Sum of oxidation
numbers
Number of element 2 1 4
0
Oxidation number +1 x -2
2(+1) + 1(x) + 4(-2) = 0
2 + x - 8 = 0
x = +6
Thus;
Oxidation number of S in H2SO4 = +6

6. Azlee Chemistry 2009
(iii) Na2S2O3
Element Na S O
Sum of oxidation
numbers
Number of element 2 2 3
0
Oxidation number +1 x -2
2(+1) + 2(x) + 3(-2) = 0
2 + 2x - 6 = 0
x = 4
2
= +2
Thus;
Oxidation number of S in Na2S2O3 = +2

7. Azlee Chemistry 2009
(iv) NH3
Element N H
Sum of oxidation
numbers
Number of element 1 3
0
Oxidation number x +1
1(x) + 3(+1) = 0
x = -3
Thus;
Oxidation number of N in NH3 = -3

8. Azlee Chemistry 2009
(v) S2O3
2-
[polyatomic ion]
[refer to rules above]
Element S O
Sum of oxidation
numbers
Number of element 2 3
-2
Oxidation number x -2
2(x) + 3(-2) = -2
2x - 6 = -2
x = (6 – 2) /2
x = +2
Thus;
Oxidation number of S in S2O3
2-
= +2
(vi) NH4
+
: ammonium ion [polyatomic ion]
Element N H
Sum of oxidation
numbers
Number of element 1 4
+1
Oxidation number x +1
x(1) + 4(+1) = +1
x + 4 = +1
x = 1 - 4
x = -3
Thus;
Oxidation number of N in NH4
+
= -3

9. Azlee Chemistry 2009
Exercises;
Calculate the oxidation numbers for the underlined
elements.
i. KMnO4 potassium manganate(VII)
ii. MnO4
-
iii. K2Cr2O7 Potassium dichromate(VI)
iv. Cr2O7
2-

10. Azlee Chemistry 2009
Answer:
i. KMnO4
Element K Mn O
Sum of oxidation
numbers
Number of element 1 1 4
0
Oxidation number +1 x -2
1(+1) + 1(x) + 4(-2) = 0
1 + x – 8 = 0
x = +7
Thus;
Oxidation number of Mn in KMnO4 = +7

11. Azlee Chemistry 2009
ii. MnO4
-
Element Mn O
Sum of oxidation
numbers
Number of element 1 4
-1
Oxidation number x -2
1(x) + 4(-2) = -1
x – 8 = -1
x = +7
Thus;
Oxidation number of Mn in KMnO4 = +7

12. Azlee Chemistry 2009
iii. K2Cr2O7
Element K Cr O
Sum of oxidation
numbers
Number of element 2 2 7
0
Oxidation number +1 x -2
2(+1) + 2(x) + 7(-2) = 0
2 + 2x – 14 = 0
2x = 12
2 x = 12 / 2
x = +6
Thus;
Oxidation number of Cr in Cr2O7
2-
= +6

13. Azlee Chemistry 2009
iv. Cr2O7
2-
Element Cr O
Sum of oxidation
numbers
Number of element 2 7
-2
Oxidation number x -2
2(+1) + 2(x) + 7(-2) = 0
2 + 2x – 14 = 0
2x = 12
2 x = 12 / 2
x = +6
Thus;
Oxidation number of Cr in Cr2O7
2-
= +6