Elements of group15 have 5 valence electrons in their valence shell. So they exhibit various oxidation state from – 3 to +5 as discussed below.
Negative oxidation state:- These elements has 5 electrons in the valence shells and require 3 more electrons to acquire nearest noble gas configuration but gain of 3 electrons is energetically not favoured. But since nitrogen is most electronegative element and smallest atom it forms (nitride) ion by gaining 3 electrons, other elements form covalent compounds with metals and show an oxidation state of – 3 with metals
Tendency of elements to exhibit – 3 oxidation state decreases on moving down the group from P to Bi
Positive oxidation State :- All elements show positive oxidation state from +3 and +5 on moving down the group. Stability of +5 oxidation state decrease while that of +3 increase due to inert pair effect (The inability of inner s- electrons to take part in bonding)
Because all elements cannot lose all 5 valence electrons, they do not form ions and all compounds of group 15 having +5 oxidation state i.eare covalent compounds.
Disproportionation :- In case of nitrogen, all oxidation states from +1 to +4 tend to disproportionate in acid solution for ex –
Similarly in case of phosphorous all intermediate oxidation states disproportionate into +5 and -3
disproportionation because of inert pair effect.