CBSE Class 11-science Answered

There are several rules for assigning the oxidation number to an element.

• The oxidation number of an atom in the elemental state is zero.
  Example: Cl₂ and Al both are 0
• The oxidation number of a monatomic ion is equal to its charge.
  Example: In the compound NaCl, the sodium has an oxidation number of +1 and the chlorine is -1.
• The algebraic sum of the oxidation numbers in the formula of a compound is zero.
  Example: the oxidation numbers in the NaCl above add up to 0
• The oxidation number of hydrogen in a compound is +1, except when hydrogen forms compounds called hydrides with active metals, and then it is -1.
  Examples: H is +1 in H₂O, but -1 in NaH (sodium hydride).
• The oxidation number of oxygen in a compound is -2, except in peroxides when it is -1, and when combined with fluorine. Then it is +2.
  Example: In H₂O the oxygen is -2, in H₂O₂ it is -1.
• The algebraic sum of the oxidation numbers in the formula for a polyatomic ion is equal to the charge on that ion.
  Example: In the sulfate ion, SO₄^2-, the oxidation numbers of the sulfur and the oxygens add up to 2-. The oxygens are -2 each, and the sulfur is +6.