Anomalous Behavior of Nitrogen:- Nitrogen, the first member of group 15, differs from others group members because of :-
i)             Small size of N atom.
ii)           High value of electronegativity of N atom and high ionization energy.
iii)          Absence of d-orbitals in the valency shell.
iv)          Tendency of form multipole bonds.
The properties in which nitrogen differs from other members of 15 groups are listed below.
i)             Nitrogen is gas while others members are solids.
ii)           Nitrogen molecule is diatomic while other elements from teratomic molecules such as p4, As4 and Sb4.
iii)          The catenation property is more pronounced in nitrogen.
iv)          Nitrogen forms five oxides of monomeric nature. Others can form at the most three types of oxides, X4O6, X4O10 of dimeric nature. N2O4 exists in dimeric from and is diamagnetic.
v)           Hydride of nitrogen is stable while the hydrides of other elements are not stable and acts as reducing agent. Hydrogen bonding is present in ammonia but not present in other hydrides.
vi)          Expect NF3, the halides of nitrogen are unstable and explosive. The halides of other elements are stable. Unlike P, as and Sb, nitrogen does not form pentahalides.
vii)        Nitrogen can form trinegative ion N3-. This tendency is less in P but absent in other elements.
						

							
Due to small size absence of d orbital and high electronegativity
It is gas other member of the group is solid 
Absence of d orbital