There are electrons around the nucleus. The electrons are divided into Electron shell according to their energy. Each Electron shell is divided into sub Electron shell. There are orbits in the sub Electron shell. Each orbit has only two electrons with opposite spin directions. The existence areas of these two electrons can be considered to be basically the same

When an external magnetic field appears, all electrons in Orbital motion will be affected by the magnetic force, and their rotating circles will be offset by a certain degree, which will produce a small additional magnetic field, that is, magnetic moment. But if another electron in the same orbital with opposite spin also produces an opposite additional magnetic field, the magnetic moment of the previous electron will be offset

So the more single electron orbitals, the greater the magnetic moment

Such molecules include H2O, SiO2, and AlCl3

When the magnetization M is negative, the solid is Diamagnetism. Metals such as Bi, Cu, Ag, Au have this property. In the external magnetic field, the Magnetic flux density inside such magnetized medium is less than the Magnetic flux density M in vacuum. The magnetic moment of atoms (ions) of Diamagnetism materials should be zero, that is, there is no permanent magnetic moment. When the Diamagnetism material is put into the external magnetic field, the external magnetic field will change the electron orbit and induce a magnetic moment opposite to the direction of the external magnetic field, which is shown as Diamagnetism. So Diamagnetism comes from the change of electronic orbital state in atoms. The Diamagnetism of Diamagnetism materials is generally weak, and the Magnetic susceptibility H is generally about -10-5, which is negative