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Oxidation

When an atom/ion/molecule loses electrons, oxidation of the species takes place, such a molecule is termed as reductant.

or

When oxidation number of an atom increases in a reaction, it is said to be oxidised.

Oxidation Number / Oxidation State

The real or hypothetical charge present over an element is called oxidation number. Whereas oxidation state defines charge on one atom but oxidation number refers to charge present on all atoms of one element in a compound.

Certain features of oxidation number

  • The pure oxidation number is always an integer, but the mathematical average may be in fraction.
  • Oxidation number may be positive as well as negative.
  • Oxidation number of I(A) group elements is +1, II(A) group element +2, and in III(A) group Al & B have +3 oxidation number and rest are variable.
  • The molecules which exist in free state, always have zero oxidation number (NH3, H2O etc).
  • Oxidation state of hydrogen is always +1, but when hydrogen is directly attached to metal (metal hydride) then its oxidation no. is always –1.
  • The oxidation No. of oxygen is –2 but in peroxide compounds its oxidation No. is –1.
  • Oxidation No. of oxygen is +ve when it is directly attached to fluorine.
  • · In superoxide compounds oxidation number of oxygen  –1/2.

    • che2.1

  • The sum of oxidation No. in neutral species is zero.
  • In halogens oxidation number of F is ¬–1 because F has maximum electronegative value in periodic table.

Let us now focus our attention on the calculation of the oxidation number

  • Break a molecule into its atoms
  • Now valencies of all the atom’s are added
  • This is than equated to the total formal charge on the molecule
  • The total formal charge on a neutral molecule is taken as zero. For a charged molecule, total formal charge is taken equal to charge on cation/anion.

Important
For finding out formal charge on an atom, hypothetically break all bonds to that atom. The e-pair of bond goes to more electronegative atom. After this exercise total charge left on central atom would be called the formal charge on that atom.

Illustrations

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