how to calculate oxidation number
Answers
Answer:
The oxidation number of a free element is always 0.
The oxidation number of a monatomic ion equals the charge of the ion.
The oxidation number of H is +1, but it is -1 in when combined with less electronegative elements.
The oxidation number of O in compounds is usually -2, but it is -1 in peroxides.
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Explanation:
The oxidation number of a free element is always 0.
The oxidation number of a monatomic ion equals the charge of the ion.
The oxidation number of
H
is +1, but it is -1 in when combined with less electronegative elements.
The oxidation number of
O
in compounds is usually -2, but it is -1 in peroxides.
The oxidation number of a Group 1 element in a compound is +1.
The oxidation number of a Group 2 element in a compound is +2.
The oxidation number of a Group 17 element in a binary compound is -1.
The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.
The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
EXAMPLE:
What is the oxidation number of
Cr
in
CrCl
3
?
Solution:
We use what rules we can to determine the oxidation numbers.
Rule 7 states that the oxidation number of Cl is -1.
We write the oxidation number of the element above its symbol and the total for 3 Cl atoms below the symbol.
This gives
Cr
-1
Cl
3
m
m
m
m
m
m
l
l
-3
m
m
.
Rule 8 states the numbers along the bottom must add up to zero. So the number under
Cr
must be +3.
This gives
Cr
-1
Cl
3
m
m
m
m
m
+3
l
l
-3
m
m
.
There is only one
Cr
atom, so its oxidation number is +3.
This gives
+
3
Cr
-1
Cl
3
m
m
m
m
m
+3
l
l
-3
m
m
.
The oxidation number of
Cr
in
CrCl
3
is +3.
Here is a chart showing the oxidation numbers of the atoms in some common elements and compounds.