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Oxidation Numbers
The oxidation number of an element is a way of keeping track of the number of electrons that are gained or lost when it combines with other atoms to form a molecule.
N.B. The same element can have a different oxidation number in different compounds.
E.g. The oxidation number of H in HCl is +1.
The oxidation number of H in H 2 is 0.
The possible oxidation numbers of an atom can be derived from its ground state electronic configuration.
The actual oxidation number of an atom depends on the other atoms in the molecule.
Rules for Oxidation Number Determination
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Rule |
Example |
| 1) |
The oxidation number of the most electronegative element in the molecule is negative. |
NaCl, Na > 0 |
| 2) |
The oxidation number of the most electropositive element in the molecule is positive. |
NaCl, Cl < 0 |
| 3) |
The oxidation number of any free element is zero. |
Na(s), H2(g) = 0 |
| 4) |
The oxidation number of a monoatomic ion is the same as its charge. |
Na+ = +1
Ca2+ = +2 |
| 5) |
The sum of the oxidation numbers for a neutral molecule is zero. |
H2O = 0 |
| 6) |
The sum of the oxidation numbers for an ionic species is equal to its charge. |
NO3- = -1 |
| 7) |
The oxidation number of alkali metals in a compound is +1. |
LiCH3, Li = +1 |
| 8) |
The oxidation number of the alkaline earth metals in a compound is +2. |
MgSO4, Mg = +2 |
| 9) |
The oxidation number of fluorine in a compound is -1. |
F = -1 |
| 10) |
The oxidation number of hydrogen is +1 in most compounds but is -1 in metal hydrides. |
H2O, H = +1
LiAlH4, H = -1 |
| 11) |
The oxidation number of oxygen is -2 in most compounds, but -1 in peroxides and - ½ superoxides.
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H2O, O = -2
HOOH, O = -1
HO[O]nOH, O = -½ |
Examples:
a) The oxidation number of Mn in MnO2.
Mn is the more electropositive element so its oxidation number is positive.
Oxygen is the more electronegative element and normally has an oxygen number of -2.
Neutral molecule so,
Oxidation Number of Mn + 2(-2) = 0
=> Oxidation Number of Mn = 4
b) The oxidation number of N in KNO3.
K has an oxidation number of 1.
O has an oxidation number of -2
Neutral molecule so,
+1 + Oxidation Number of N + 3(-2) = 0
=> Oxidation number of N = 6 - 1 = 5
Practice Problems:
What are the oxidation numbers of:
a) O in KO2
b) H in NaH
c) Fe in Fe3O4
d) Fe in Fe(NH4)2(SO4)2
Answers
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