Explain paramagnetism and diamagnetism.

(N/A) $(i)$ Paramagnetism: Substances that contain one or more unpaired electrons are weakly attracted by an external magnetic field. These substances are magnetized in the direction of the magnetic field and lose their magnetism when the field is removed. Examples: $O_2, Cu^{2+}, Fe^{3+}, Cr^{3+}$.
$(ii)$ Diamagnetism: Substances in which all electrons are paired (completely filled orbitals) are weakly repelled by an external magnetic field. They are weakly magnetized in the direction opposite to the applied magnetic field. The pairing of electrons cancels out their individual magnetic moments. Examples: $H_2O, NaCl, C_6H_6$.