Oxidation States
Assigning oxidation numbers
1
Assigning Oxidation Numbers
• An “oxidation number” is a positive or
negative number assigned to an atom
to indicate its degree of oxidation or
reduction.
• Generally, a bonded atom’s oxidation
number is the charge it would have if
the electrons in the bond were
assigned to the atom of the more
electronegative element
Rules for Assigning Oxidation Numbers
1. The oxidation number of any
uncombined element is zero.
• Examples: all atoms in sodium
Na, oxygen O2, phosphorus P4,
and sulfur S8.
Rules for Assigning Oxidation Numbers
2. The oxidation number of a
monatomic ion equals its
charge.
0
0
1
1
2 Na  Cl 2  2 Na Cl
4
Rules for Assigning Oxidation Numbers
3. The oxidation number of fluorine is
ALWAYS -1
4. oxygen usually has and oxidation
number of -2
Exceptions:
• In peroxides, such as H2O2 oxygen
has an oxidation number of -1.
• In compounds with fluorine, such as
OF2, oxygen’s oxidation number is +2.
Rules for Assigning Oxidation Numbers
5. Hydrogen has and oxidation number of
+1.
Exceptions:
In metal hydrides, like NaH, hydrogens
oxidation number is -1.
1
2
H2O
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Rules for Assigning Oxidation Numbers
6. The sum of the oxidation numbers of the
atoms in a neutral compound must
equal 0.
1
2
H2O
2(+1) + (-2) = 0
H
O
2
2 1
Ca (O H ) 2
(+2) + 2(-2) + 2(+1) = 0
Ca
O
H
Rules for Assigning Oxidation Numbers
7. The sum of the oxidation numbers in
the formula of a polyatomic ion is equal
to its ionic charge.
? 2
N O3

X + 3(-2) = -1
N
O
thus X = +5
? 2
S O4
2
X + 4(-2) = -2
S
O
thus X = +6