Nitrogen is inert molecule because in nitrogen there exist a triple bond betwwn the two nitrogen atoms.
This triple bond constituted of one sigma and two pi bonds i.e.
P_x -P_x overlap to form sigma bond.
P_y-P_y and P_z-P_z overlaps to form pi bond.
The intermolecular forces in this are weak Van der Walls forces. So, the diatomic molecules exist freely.Because of the triple bond,the bond energy is very high.The bond dissociation energy of nitrogen is 945.4 kJ/mol.Due to this high bond dissociation energy, nitrogen is apparently inactive under normal conditions.This is why nitrogen is unreactive gas.