Why is nitric oxide paramagnetic in gaseous state but the solid obtained on cooling it is diamagnetic?
paramagnetic: the species which contain unpaired electrons are known as paramagnetic.
Diamagnetic: the species which all paired electrons are called diamagnetic.
Nitric acid in gaseous state exists in monomer form. It consists of only one unpaired electron therefore it is paramagnetic in nature.
However in solid state it exists as dimer (N2O2). There is no unpaired electron in its dimer form, therefore it is diamagnetic in nature.