Question

What are the rules for finding oxidation number?

Answer 1

Oxidation Number Rules

1.The oxidation number for an atom in its elemental form is always zero.

2.A substance is elemental if both of the following are true:
only one kind of atom is present
charge = 0
Examples:
S8: The oxidation number of S = 0
Fe: The oxidation number of Fe = 0
The oxidation number of a monoatomic ion = charge of the monatomic ion.
Examples:
Oxidation number of S2- is -2.
Oxidation number of Al3+ is +3.

3.)The oxidation number of all Group 1A metals = +1 (unless elemental).

4.)The oxidation number of all Group 2A metals = +2 (unless elemental).

5.) Hydrogen (H) has two possible oxidation numbers:
+1 when bonded to a nonmetal
-1 when bonded to a metal

6.) Oxygen (O) has two possilbe oxidation numbers:
-1 in peroxides (O22-)....pretty uncommon
-2 in all other compounds...most common

7.) The oxidation number of fluorine (F) is always -1.

8.) The sum of the oxidation numbers of all atoms (or ions) in a neutral compound = 0.

9.) The sum of the oxidation numbers of all atoms in a polyatomic ion = charge on the polyatomic ion.