Oxidation Numbers Worksheet
Directions: Use the Rules for Assigning Oxidation Numbers to determine the oxidation number assigned
to each element in each of the given chemical formulas.
Formula
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
Cl2
ClNa
Na+
O2
N2
Al+3
H2O
NO3NO2
Cr2O72KCl
NH3
CaH2
SO42-
Element and Oxidation
Number
Cl
Cl
Na
Na
O
N
Al
H
O
N
O
N
O
Cr
O
K
Cl
N
H
Ca
H
S
O
Formula
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
Na2O2
SiO2
CaCl2
PO43MnO2
FeO
Fe2O3
H2O2
CaO
H2S
H2SO4
NH4Cl
K3PO4
HNO3
KNO2
Element and Oxidation Number
Na
Si
Ca
P
Mn
Fe
Fe
H
Ca
H
H
N
K
H
K
O
O
Cl
O
O
O
O
O
O
S
S
H
P
N
N
O
Cl
O
O
O
Rules for Assigning Oxidation Numbers
1. The oxidation number of any uncombined element is 0.
2. The oxidation number of a monatomic ion equals the 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 (when it is +2), or it is in a
peroxide (such as H2O2 or Na2O2), when it is -1.
6. The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a
metal, in which case it is -1.
7. In compounds, the elements of groups 1 and 2 as well as aluminum have oxidation numbers of
+1, +2, and +3 respectively.
8. The sum of the oxidation numbers of all atoms in a neutrals compound is 0.
9. The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion.
Answer Key
1.
Cl:0
2.
Cl:13.
Na:0
4.
Na:1+
5.
O:0
6.
N:0
7.
8.
9.
10.
11.
12.
Al:3+
H:1+
N:5+
N:4+
Cr:6+
K:1+
O:2O:2O:2O:2Cl:1-
13.
14.
15.
16.
17.
18.
N:3- H:1+
Ca:2+ H:1S:6+ O:2Na:1+ O:1Si:4+ O:2Ca:2+ Cl:1-
19.
20.
21.
22.
23.
24.
P:5+
Mn:4+
Fe:2+
Fe:3+
H:1+
Ca:2+
O:2O:2O:2O:2O:1O:2-
25.
26.
27.
28.
29.
30.
H:1+
H:1+
N:3K:1+
H:1+
K:1+
S:2S:6+
H:1+
P:5+
N:5+
N:3+
O:2Cl:1O:2O:2O:2-