When does paramagnetism occur? How does it differ from diamagnetism?

Materials that display paramagnetism are mildly attracted by the magnetic field, but do not retain the magnetism, as the electron shells return to their original configuration after the field is removed.

Elements that display paramagnetism include tantalum, lithium and sodium.

Paramagnetism is often compared to ferromagnetism and diamagnetism. Both of these are defined in the information following.

There are images and information here:
https://www.nde-ed.org/EducationResources/CommunityCollege/MagParticle/Physics/MagneticMatls.htm

The "spin" quantum number can either be
#m_s=1//2# or
#m_s=-1//2#

[Note: spin is in quotation marks because you can't really visualize an electron "spinning" on it's axis; that's not really what it's doing]

So an electron could either have a spin up, #uarr# or spin down, #darr#. Moving charges produce magnetic fields. So an electron is just a tiny magnet. When you have an unpaired electron, the magnetic fields of all the electrons add together to overall not cancel out. That situation is what we call paramagnetic.

When you have two electrons with opposite spins, #uarr darr#, the magnetic fields of those electrons cancel each other. This situation is called diamagnetic.

There is more information available here:
https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/electron-configurations-jay-sal/v/paramagnetism-and-diamagnetism