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/molecules of diamagnetic substances have electons that can be grouped in pairs with spins in opposite sense. In paramagnetic materials, atoms/molecules have excess electrons which spin in the same sense.
Elements with even atomic number have electons which can be grouped into pairs with spins in opposite sense. When external field is applied, the electons in each pair get equal magnetic moments opposite to each other, making the resultant magnetic moment zero. Therefore, such elements are more likely to be diamagnetic.