chemistry 12th class cbse notes

Negative and Positive oxidation state:

  • The common oxidation states of these elements are – 3, + 3 and + 5.
  • The tendency to exhibit – 3 oxidation state decreases down the group, bismuth hardly forms any compound in –3 oxidation state.
  • The stability of + 5 oxidation state decreases down the group. The only well-characterized Bi (V) compound is BiF5.
  • The stability of + 5 oxidation state decreases and that of +3 state increases (due to inert pair effect) down the group.
  • Nitrogen exhibits +1, + 2, + 4 oxidation states also when it reacts with oxygen. Phosphours also shows + 1 and + 4 oxidation states in some oxoacids.


  • In the case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution. For example,

Oxidation States

  • Similarly, in case of phosphorus nearly all intermediate oxidation states disproportionate to +5 and –3 both in alkali and acid.
  • However, +3 oxidation states in case of arsenic, antimony and bismuth become increasingly stable with respect to disproportionation.


  • Nitrogen is restricted to a maximum covalency of 4 since only four (one s and three p) orbitals are available for bonding.
  • The heavier elements have vacant d orbitals in the outermost shell which can be used for bonding (covalency) and hence, expand their covalence as in PF6.

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