Rules for Assigning Oxidation Numbers
- The oxidation number for an atom of any free
(uncombined) element is ZERO.
- Examples: Na, Ca have zero oxidation
number.
- The oxidation number of an element in self-combination is always ZERO.
- Examples: H2, O2,
P4have zero oxidation number.
- In most hydrogen containing compounds,
oxidation number of hydrogen is + 1.
- (Exception is when H combines with alkali metals or alkaline earth to
form hydrides of metals such as: NaH, LiH, CaH2. Then, the
oxidation number of H is -1).
- In compounds involving the alkali
metals, the elements are assigned oxidation number of +1.
- In compounds involving the alkaline
earth metals, the elements are asasigned oxidation number of +2.
- Oxygen is usually assigned
an oxidation number of -2 for
oxides. It has an oxidation
number of -1 in peroxides
(H2O2).
- Fluorine always has oxidation
number of -1 in compounds. The
other elements in that group is usually -1 in compounds with elements of low
electronegativity.
- The sum of oxidation numbers of all the atoms in
the formula for a neutral compound is ZERO.
- The sum of oxidation numbers of an ion or complex
ion is the same as the charge on that ion.
- Negative oxidation number in compounds of two unlike atoms are assigned
to the more electronegative atom.
| Examples: |
Oxidation numbers |
| HCl |
H = +1, Cl = -1 |
| H2O |
H = +1, O = -2 |
- In combinations of nonmetals not involving hydrogen and oxygen, the
nonmetal that is more electronegative is considered negative.
| Examples: |
Oxidation numbers |
| CCl4 |
C = +4, Cl = -1 |
| SF6 |
S = +6, F = -1 |
| CS2 |
C = +4, S = -2 |
