What is the basic difference between the atom or molecule of a diamagnetic and a paramagnetic material? Why are elements with even atomic number more likely to be diamagnetic?

Atoms or molecules of a diamagnetic substance contain even number of electrons and these electrons form the pairs of opposite spin. While the atoms or molecules of a paramagnetic substance have excess of electrons spinning in the same direction.
The elements with even atomic number Z has even number of electrons on its atoms or molecules so they are more likely to form electrons pairs of opposite spin and hence are more likely to be diatomic.