CBSE Class 11-science Answered

Oxidation number of an element is defined as the charge which its atom has or appears to have when all the other atoms from the molecule are assumed to be removed as ions by counting the shared electrons with more electronegative atom.

General rules for finding the oxidation number of an atom: 

• Oxidation number of atoms in free elemental state (like H₂, Na, O₂, Ag etc) = 0
• Oxidation number of monoatomic ions = Charge on them ( For example : Halogens (like Fluorine, chlorine) = -1, Na⁺ = +1, Ca⁺² = +2 etc)
• Oxidation number of oxygen = -2. In peroxides and superoxides, oxidation number of oxygen is (-1). In Oxygen diflouride (F₂O), oxidation number of oxygen is +2. In dioxygen diflouride (F₂O₂), oxidation number of oxygen +1.

• Oxidation number of hydrogen = +1; however in metal hydrides it is (-1)
• Sum of Oxidation number of all the atoms in molecules = 0
• Sum of Oxidation number of atoms in polyatomic ions is equal to overall charge on them.