If all the electrons are paired, the molecule is diamagnetic, while if one or more electron in a molecule is unpaired, the species is paramagnetic. O22− is not paramagnetic because in O22− all the electrons are paired, i.e. Configurationof O22− (Total electrons =18 ) (σs)2,(σls)2*,(σ2s)2,(σ2s)2*,(σ2pz)2,(π2py)2,(π2px)2) (π2px)2*,(π2py)2* Hence, O22− is diamagnetic.