Name:_____________________________
Worksheet – ASSIGNING OXIDATION NUMBERS
Oxidation Number Rules:
1. The oxidation number of any pure element is 0.
2. The oxidation number of a monatomic ion equals that charge on the ion.
3. The more electronegative element in a binary compound is assigned the number equal to the
charge it would have if it were an ion.
4. The oxidation number of fluorine in a compound is always -1.
5. Oxygen has an oxidation number of -2 unless it is combined with F, in which it is +1 or +2, or it
is in peroxide (such as H2O2 or Na2O2), in which it is -1.
6. Hydrogen is +1, unless combined with a metal, and then it is -1.
7. In compounds, Group 1 is +1, Group 2 is +2, and Aluminum is +3.
8. The sum of the oxidation numbers of all atoms in a neutral compound is 0.
9. The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion.
Part A: In the following questions, give the oxidation number of the indicated atoms/ion:
1. N in N2O3
__________
2. S in H2SO4
3. C
__________
__________
5. Na in NaCl
__________
6. H in H2O
__________
7. Ba in BaCl2
__________
17. Mn in MnO2
__________
__________
19. Mg2+
__________
20. Cl-
__________
21. O2
__________
22. P4
__________
8. N in NO2-
__________
23. Na in Na2S
9. S in Al2S3
__________
24. S in H2S
10. S in HSO4-
__________
18. S in SO32-
__________
4. C in CO
16. C in CH4
__________
25. Ca2+
__________
__________
__________
11. Cl in Fe(ClO2)3
__________
26. C in CN-
__________
12. Fe in Fe(ClO2)3
__________
27. H in OH-
__________
13. N in NO3-
__________
28. Mn in KMnO4
14. Cu2+
__________
29. I in Mg(IO3)2
15. Zn2+
__________
30. C in C2O42-
__________
__________
__________
Part B: Identify the species being oxidized and reduced in each of the following reactions and write
their half reactions:
1. 2 Cr+ + Sn4+  2 Cr3+ + Sn2+
Oxidized:
Reduced:
2. 3 Hg2+ + 2 Fe (s)  3 Hg2 + 2 Fe3+
Oxidized:
Reduced:
3. 2 As (s) + 3 Cl2 (g)  2 AsCl3.
Oxidized:
Reduced:
4. Zn + Cu2+  Zn2+ + Cu
Oxidized:
Reduced:
5. C + H2SO4  CO2 + SO2 + H2O
Oxidized:
Reduced:
6. HNO3 + HI  NO + I2 + H2O
Oxidized:
Reduced:
7. KMnO4 + HCl  MnCl2 + Cl2 + H2O + KCl
Oxidized:
Reduced:
8. Sb + HNO3  Sb2O3 + NO + H2O
Oxidized:
Reduced:
2