Discuss the anomalous behaviour of nitrogen.

(N/A) The anomalous behaviour of nitrogen is due to:
$(i)$ Small size
$(ii)$ High electronegativity
$(iii)$ High ionization enthalpy
$(iv)$ Absence of $d$-orbitals
$1$. Nitrogen exists as a diatomic $(N_2)$ gaseous molecule due to its ability to form $p\pi-p\pi$ multiple bonds,whereas other elements of the group exist as solids due to larger atomic size and inability to form effective $p\pi-p\pi$ bonds.
$2$. Nitrogen has a unique ability to form $p\pi-p\pi$ multiple bonds with itself and with small,highly electronegative elements like oxygen and carbon. Heavier elements form single bonds ($P-P$,$As-As$,$Sb-Sb$) because their atomic orbitals are too large and diffuse for effective overlapping.
$3$. Catenation tendency is weaker in nitrogen because the $N-N$ single bond is weaker than the $P-P$ bond due to strong interelectronic repulsions between the lone pairs of nitrogen atoms.
$4$. Nitrogen cannot expand its covalency beyond $4$ due to the absence of $d$-orbitals in its valence shell,preventing the formation of pentavalent compounds like $PCl_5$.
$5$. Nitrogen cannot form $d\pi-p\pi$ bonds,unlike heavier elements which can form such bonds with transition metals or oxygen (e.g.,$R_3P=O$).
$6$. Trihalides of nitrogen,except $NF_3$,are unstable,whereas trihalides of other group-$15$ elements are stable.