Oxidation Number

Oxidation Number

The charge present on atom in molecule or ion is called oxidation number. It may be zero, positive or negative.

Rules for determination of oxidation number

(i) Oxidation number of an atom in free state is zero.

(ii) Oxidation number of alkali metals (Li, Na, K, Rb, Cs) in molecule is always +1.

(iii) Oxidation number of alkaline earth metals (Be, Mg, Ca, Sr, Ba) in a molecule is always + 2

(iv) Oxidation number of hydrogen —
.       (+ 1) hydrogen ion
     (- 1) hydride ion

(v) Oxidation number of Oxygen
      (-2) oxide
      (-1) peroxide
      -1/2 superoxide

(v) Sum of Oxidation number of atoms in a molecule is equal to zero.

(vi) Sum of oxidation number of atoms in a ion is equal to magnitude of charge with sign.

Oxidation Number of Mn in KMnO₄ :
      Let O.N. of Mn = x
      1 + x + (-2) × 4 = 0
      1 + x - 8 = 0
      x = + 7

Oxidation Number of Cr in K₂Cr₂O₇ :
      Let O.N. of Cr = x
      1 × 2 + x × 2 + (-2) × 7 = 0
      1 + 2x - 14 = 0
      x = + 6

Oxidation Number of C in C1₂H₂₂O₁₁ :
      Let O.N. of C = x
      x × 12 + 1 × 22 + (-2) × 11 =0
      12x + 22 - 22 = 0
      x = 0