Which species is diamagnetic?

Question

a) NO

b) N2 2

c) O2

d) O2 2-

The correct answer is D, but can someone please explain why?

What exactly does it take for a compound to be diamagnetic?

O2 is considered paramagnetic but why?

Answer 1

A compound will be diamagnetic if all electrons are paired within their molecular orbitals.  If one or more electrons are unpaired, the compound is diamagnetic.

NO and O2 have unpaired electrons.  O2^2- is like F2, so it has no unpaired electrons and will be diamagnetic.

I'm not sure what you mean by N2 2.  If you mean N2^2+, i.e. the doubly charged positive ion of N2, that should be diamagnetic too since it is isoelectronic with C2, which is diamagnetic.  There's something wrong with that one.  The person who wrote the question may not have the correct order of the sigma and pi orbitals in these diatomics.  See the source reference for molecular orbitals of diatomics.