Chemistry II
Dry Lab 1: Oxidation Numbers
Purpose: To become familiar with common oxidation numbers of various elements. To
determine the oxidation number of an atom given the chemical formula for a compound or
polyatomic ion.
Introduction:
When elements combine to produce a compound, each element is thought of as acquiring an
“apparent” charge. The apparent charge, the charge an atom would have if the electrons in the
bond were assigned to the more electronegative element, may be either positive or negative. It is
called the oxidation number (or oxidation state) of the element in the compound.
Oxidation numbers are very handy bookkeeping devices for keeping track of what happens to
electrons when various elements combine to form compounds. By remembering a relatively few
generalizations concerning oxidation numbers, the correct chemical formulas for a large number
of compounds can be written. It is unnecessary to simply memorize disconnected chemical
formulas. As in any system, some rules of accounting must be followed. Some rules for
assigning oxidation numbers are:
1. Any element in the free state (not combined with any other elements) has an oxidation number
of zero, regardless of the complexity of the molecule in which it occurs. Each atom in Ne, O2, P4,
and S8 has an oxidation number of zero.
2. The oxidation number of any mono-atomic ion equals the charge on the ion. The ions Ca2+,
Fe3+, and Cl- have oxidation numbers of 2+, 3+, and 1- respectively.
3. Oxygen in compounds is assigned an oxidation number of 2- (except for a 1- in peroxides,
e.g., H2O2 and 2+ in OF2.) The oxidation number of oxygen is 2- in FeO, Fe2O3, KMnO4, and
KIO3.
4. Hydrogen in compounds is assigned an oxidation number of 1+ (except for a 1- in metal
hydrides, e.g., NaH). Its oxidation number is 1+ in HCl, NaHCO3, and NH3.
5. Some elements exhibit only one common oxidation state in certain types of compounds.
a. Group 1 elements always have an oxidation number of 1+ in compounds.
b. Group 2 elements always have an oxidation number of 2+ in compounds.
c. Boron and aluminum always possess an oxidation number of 3+ in compounds.
d. In binary compounds with metals, the non-metallic elements of Group 16 usually
exhibit an oxidation number of 2-.
e. In binary compounds with metals, the elements of Group 17 have an oxidation number
of 1-.
6. The charge for a polyatomic ion is considered its “net” oxidation number. The polyatomic
ions SO42-, NO3-, and PO43- have “net” oxidation numbers of 2-, 1-, and 3- respectively.
7. In assigning oxidation numbers in a compound, the elements closest to fluorine (the most
electronegative element) in the periodic table are always assigned the negative oxidation number.
In the compound, P4O10, oxygen has the negative oxidation number of 2-.
8. a. For neutral compounds the sum of the oxidation numbers of all atoms in the compound
must equal zero.
Example #1 Na2S
The sum of the oxidation numbers equals zero.
2 Na atoms, 1+ for each (rule 5a) = 2+
1 S atom, 2- (rule 5d)
= 2Sum of oxidation numbers (2+ + 2-) = 0
b. For charged polyatomic ions the sum of the oxidation numbers of the elements must equal
the charge on that ion.
Example #2 CO32The sum of the oxidation numbers equals 2-.
3 O atoms, 1- for each (rule 3)
= 61 C atom, which must be 4+
= 4+
so that (6-) + (4+) = 2-, the charge on the CO32- ion.
9. Some chemical elements show more than one oxidation number, depending upon the
compound. The rules above may be used to determine their values. Consider the compounds
FeCl2 and FeCl3. Since the chloride atom has an oxidation number of 1- when combined with a
metal (rule 5e), the oxidation number of iron are 2+ for FeCl2 and 3+ for FeCl3.
Names ____________________________________
____________________________________
Date _________________________
Chemistry II
Dry Lab 1: Oxidation Numbers
Procedure:
Now let’s apply these rules to a few exercises. Determine the oxidation number on the bold,
italicized atom in each of the following compounds and ions.
1. CO
2. CO2
3. IF7
4. Li3N
5. K2S
6. SnCl4
7. BaNO4
8. Ca(NO2)2
9. Fe2O3
10. HClO3
11. HClO
12. PO43-
13. BO32-
14. ZrI3-
15. AsO33-
16. HCO3-
17. CO32-
18. NH4+
19. P3O105-
20. S2O32-
21. PCl3
22. POCl3
23. SO3
24. SCl2
25. K2CrO4
26. MnO2
27. K2MnO4
28. Mn2O7
29. P2S5
30. NaBrO3