Question

Explain the anomalous behaviour of oxygen

Answer 1

Oxygen is diatomic gas  While others are complex solid.  

Oxygen molecule is paramagnetic in nature  and has two unpaired electron.  

All other are  Diamagnetic.

N2O4 is diamagnetic in nature with even paired of electrons. Same properties work for nitrogen dioxide, NO2 means diamagnetic in nature where there are two terminal oxygens, in this each electron is contribute to two σ bonds.