Why does nitrogen show anomalous properties with respect to other elements in group 15?

Absence of vacant d-orbitals

Due to the lack of unoccupied d-orbitals, nitrogen differs from the other elements of group 15 in terms of its physical characteristics. The smaller size, highest electronegativity, and highest ionisation energy of the nitrogen atom relative to all the elements in group 15 further contribute to its abnormal features.