The key to understanding atomic magnetism was the 1922 discovery that electrons have an inherent magnetic dipole moment. Perhaps this shouldn’t be surprising. An electron has a mass, which allows it to interact with gravitational fields, and a charge, which allows it to interact with electric fields. There’s no reason an electron shouldn’t also interact with magnetic fields, and to do so it comes with a magnetic dipole moment. The electron’s inherent magnetic dipole moment is usually called, more simply, its magnetic moment.

An electron’s inherent magnetic moment, shown in FIGURE 29.51, is often called the electron spin because, in a classical picture, a spinning ball of charge would have a magnetic moment. This classical picture is not a realistic portrayal of how the electron really behaves, but its inherent magnetic moment makes it seem as if the electron were spinning. While it may not be spinning in a literal sense, an electron really is a microscopic magnet.

FIGURE 29.51 Magnetic moment of the electron.

We must appeal to the results of quantum physics to find out what happens in an atom with many electrons. The spin magnetic moments, like the orbital magnetic dipole moments, tend to oppose each other as the electrons are placed into their shells, causing the net magnetic moment of a filled shell to be zero. However, atoms containing an odd number of electrons must have at least one valence electron with an unpaired spin. These atoms have a net magnetic moment due to the electron’s spin.

But atoms with magnetic moments don’t necessarily form a solid with magnetic properties. For most elements, the magnetic moments of the atoms are randomly arranged when the atoms join together to form a solid. As FIGURE 29.52 shows, this random arrangement produces a solid whose net magnetic moment is very close to zero. This agrees with our common experience that most materials are not magnetic.

FIGURE 29.52 The random magnetic moments of the atoms in a typical solid.