Diamagnetic refers to the ability of a material to create an opposing magnetic field when exposed to a strong one. Diamagnetism is observable in substances with symmetric electronic structure (as ionic crystals and rare gases) and no permanent magnetic moment. The effect is created by a change in the orbit of electrons, which generate small currents to oppose magnetism from external sources. Coming to diamagnetic compunds, they generally have covalent bonds, and so have very weak molecule-wise polarity. Also, because of above property, the molecules are loosely bound, so each molecule takes a direction at random, and hence total polarity of compund is even lesser. This is the reason for diamagnetism and weaker magnetic properties. The diamagnetism of some materials, however, is masked either by a weak magnetic attraction (paramagnetism) or a very strong attraction (ferromagnetism).

The pairing, or lack thereof, in the atomic structure is what causes a material to behave differently when an external magnetic field is applied. To be diamagnetic, all electrons must be paired.

cause of diamagnetism