CBSE Class 11-science Answered
how to find oxidation number in a compound
a) The oxidation number for an atom in its elemental form is always zero.
A substance is elemental if both of the following are true:
- only one kind of atom is present
- charge = 0
For examples:
- In S8, the oxidation number of S = 0 and The oxidation number of Fe is 0
b) 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.
c) The oxidation number of all Group 1A metals is +1 (unless elemental).
d) The oxidation number of all Group 2A metals is +2 (unless elemental).
e) Hydrogen (H) has two possible oxidation numbers:
- +1 if bonded to a nonmetal
- -1 if bonded to a metal
f) Oxygen (O) has two possible oxidation numbers:
- -1 in peroxides (O22-) which is very uncommon
- -2 in all other compounds which is the most common
g) The oxidation number of fluorine (F) is always -1.
h) The sum of the oxidation numbers of all atoms (or ions) in a neutral compound is 0.
i) The sum of the oxidation numbers of all atoms in a polyatomic ion = charge on that polyatomic ion.