Determination of Oxidation States of Atoms in Formulas
(Adapted from Wismer’s Study Guide for Petrucci)
OBJECTIVE: Given a formula like K2Cr2O7, determine the oxidation state of Cr from
the formula.
The following rules may appear strange, but they actually work! So it is important to
actually read through them to get the picture.
Use the following method for applying the rules:
1. Apply the rules form the top to the bottom of the list.
2. Search the list to find a rule that fits. Apply it.
3. Then start again at the top of the list to find the next rule that fits.
Oxidation State Rules:***
1. The O.S. (oxidation state) of all uncombined elements = 0.
2. The sum of the O.S in a compound = 0.
3. The sum of the O.S. in an ion = the ion charge.
4. Alkali metal ions (family IA) have O.S. = +1.
5. Alkaline earth metal atoms (family IIA) have O.S. = +2.
6. H has O.S = +1 (unless bonded to a metal then O.S. = -1)
7. F has O.S. = -1
8. O has O.S. = -2
9. Cl, Br, I (in order) have O.S. = -1
10. S, Se, Te (in order) have O.S. = -2
11. N, P, As (in order) have O.S. = -3
12. Others have O.S. = group number, or O.S. = group –8, with the more
electronegative element having the negative oxidation state.
Note *** The further down the O.S. rules you go, the more likely it is that the atom will
deviate from what you expect the O.S. to be.
Examples:
Find the oxidation state of each atom in the formula:
a)
b)
c)
d)
e)
f)
g)
h)
H2
O22NaH
K2O2
K2Cr2O7
PO33NaO2
C2H4O2
0
-1
+1, -1
+1, -1
i)
j)
k)
l)
m)
n)
o)
K3N
CaC2
IF3
NO3HClO
C2O42C2H6O