Question
Sat May 31, 2008 By: Aditya Nanda

How to count oxidation number?

Expert Reply
Sun June 15, 2008

Calculating Oxidation Number or State :
This allows for a more formal, quantitative decription of the oxidation state and is based on looking at the relative electronegativity of the atoms attached to the atom being assessed. The algebraic sum of the oxidation states must equal the charge of the molecule.

If we are looking at a central carbon atom:

  • for attached C atoms, i.e. C-C bonds electrons shared, count 0
  • for attached X atoms, i.e. C-X bonds (X more electronegative), count -1 (per bond)
  • for attached H atoms, i.e. C-H bonds (H is less electronegative than C), count +1
  • Add the total for atoms attached to the C in question, then switch the sign.

If we are looking at a generic central atom

  • for attached atom with the same electronegativity, i.e. bonding electrons shared, count 0
  • for attached X atoms, i.e. X more electronegative, count -1 (per bond)
  • for attached Y atoms, i.e. Y is less electronegative, count +1(per bond)
  • Add the total for atoms attached to the central atom question, then switch the sign.
  • Some examples are shown below:

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