Magnetism in molecules or atoms are a result of molecular spin.

You know that a moving charge results into current, and a current generates a magnetic field. As the electrons in an atom revolve, they generate magnetic field. If any orbital in an atom is paired, the magnetic field generated by the electrons cancel out, but if there is an unpaired electron, there is non zero magnetic moment.

 

If all the electrons are paired, the material is diamagnetic.

If there is less than three unpaired electron, its paramagnetic

and if three or more unpaired electron, it becomes diamagnetic.

(There are two more classes, ferrimagnetic, and antiferromgnetic, but you need not know them)

 

Each atom behaves like a tiny dipole. They are oriented randomly in any material so that their effect cancel each other to give zero magnetic moment.

But when kept in an external magnetic field, these dipoles align in the direction of the field (as you must know, equilibrium is when the dipole is parallel or anti parallel to the external field.,most of them are parallel,and not anti parallel as antiparallel is position of unstable equilibrium)

 

Now here is a fight b/w the inter atomic restoring forces and the external magnetic field.

 

If the internal forces are strong enough (or there are no unpaired electrons), they prevent the atomic dipoles to move at all, so no alignment, and hence diamagnetism.

If the internal forces are not so strong, and there is small magnetic force due to one or two unpaired electron, the dipoles align in the direction of external field, but they restore to their original position as soon as the external field is removed.

But if there are more, say three four etc no of unpaired electron, the distortion in the arrangement of the atoms is so prominent that it isnt restored.

 

You have asked bout 'molecules' but I have answered bout 'atom', because we nearly always deal with magnetism of metals and alloys, which are unimolecular, ie they exist as group or atoms in lattice and not molecules.

 

But to satisfy your inquisitiveness, at molecular level, we apply the same principles on Molecular Orbitals.... same logic...

 

Now if you get it, try to think... why oxygen molecule is paramagnetic... blue in liquid state???  (colour is also sometimes related to unpaired electrons)

 

 

(Tell me if you need more)