| Assigning Oxidation Numbers (or
state)
1. The oxidation number of an element is
zero(0).
| |
Cu(s) |
Mg(s) |
O2(g) |
S8(s) |
Hg(l) |
| Ox# |
0 |
0 |
0 |
0 |
0 |
2. For monatomic ions, the oxidation number is
the same as the charge on the ion.
| |
Na+(aq) |
Cu2+(aq) |
Fe3+(aq) |
Cl-(aq) |
S2-(aq) |
N3-(aq) |
| Ox# |
+1 |
+2 |
+3 |
-1 |
-2 |
-3 |
3. In binary compounds (or polyatomic ions),
the second element is assigned a negative
oxidation number, usually the same as the
charge on its monatomic ion.
| |
SCl2 |
SiBr4 |
CS2 |
P4S4 |
CSe2 |
CN- |
S2N+ |
| Ox# |
-1 |
-1 |
-2 |
-2 |
-2 |
-3 |
-3 |
a. F always -1.
| |
OF2 |
CF4 |
BF4- |
ClF6+ |
| Ox# |
-1 |
-1 |
-1 |
-1 |
b. O usually -2 (except peroxide -1 and
superoxide -1/2). Peroxides and superoxides
can only be determined if the cation
they are associated with has a fixed charge (H,
alkali metals or alkaline-earth
metals).
| |
H2O |
P2O5 |
VO2+ |
UO22+ |
ClO3- |
SO42- |
PO43- |
| Ox# |
-2 |
-2 |
-2 |
-2 |
-2 |
-2 |
-2 |
| |
H2O2 |
BaO2 |
Na2O2 |
KO2 |
| Ox# |
+1
-1 |
+2
-1 |
+1
-1 |
+1
-1/2 |
c. Hydrogen is -1 only when combined with
a metal, hydride ion. H is +1 when
written first (usually acids).
| |
LiH |
CaH2 |
AlH4- |
| Ox# |
-1 |
-1 |
-1 |
| |
H2O |
HCl |
H2SO4 |
H3PO4 |
| Ox# |
+1 |
+1 |
+1 |
+1 |
4. The sum of oxidation numbers in a compound
or polyatomic ion is equal to the
overall
charge on the compound (0) or the ion.
| |
SCl2 |
SiBr4 |
CS2 |
P4S4 |
CSe2 |
CN- |
S2N+ |
| Ox# |
+2
-1 |
+4
-1 |
+4
-2 |
+2
-2 |
+4
-2 |
+2
-3 |
+2
-3 |
| |
OF2 |
CF4 |
BF4- |
ClF6+ |
| Ox# |
+2
-1 |
+4
-1 |
+3
-1 |
+7
-1 |
| |
H2O |
P2O5 |
VO2+ |
UO22+ |
ClO3- |
SO42- |
PO43- |
| Ox# |
+1
-2 |
+5
-2 |
+4
-2 |
+6
-2 |
+5
-2 |
+6
-2 |
+5
-2 |
| |
LiH |
CaH2 |
AlH4- |
| Ox# |
+1
-1 |
+2
-1 |
+3
-1 |
| |
H2O |
HCl |
H2SO4 |
H3PO4 |
| Ox# |
+1 -2 |
+1 -1 |
+1
+6
-2 |
+1
+5
-2 |
These rules do work as simply with molecular
compounds with more than two types of atoms.
For example, C2H6O2,
and many other organic compounds.
|