D4.7 Unpaired Electrons and Magnetism

One way chemists have discovered information about electron configurations of ions involves magnetism. You are probably familiar with refrigerator magnets, iron magnets, or neodymium (rare earth) magnets. These exhibit ferromagnetism, a strong attraction to a magnetic field that is easily observable and sometimes can be made permanent. All substances exhibit diamagnetism, a very weak repulsion from a magnetic field that can only be observed in extremely large magnetic fields. Because diamagnetism is so weak, it is usually too small to notice.

In addition to diamagnetism, some substances also exhibit paramagnetism, an attraction to a magnetic field that is not as strong as ferromagnetism but much stronger than diamagnetic repulsion. Paramagnetism arises due to the presence of unpaired electrons. Each electron has a tiny magnetic field; that is, each electron is a tiny magnet. When a macroscopic magnet comes near a paramagnetic substance, most of the unpaired electron magnets align with the magnetic field, causing the substance to be attracted to the magnet (Figure: Paramagnetism).

Figure: Paramagnetism. A. In the absence of a magnetic field, unpaired-electron magnetic moments (arrows) are oriented randomly in all directions. B. In a magnetic field, magnetic moments orient in the same direction as the field, except for a few that orient in the opposite direction. The substance is attracted to a magnet, but not as strongly as a ferromagnetic substance. When the magnetic field is removed, the electron magnetic moments revert to a random arrangement similar to A.

Figure: Ferromagnetism shows how ferromagnetism differs from paramagnetism.

Figure: Ferromagnetism. A. In a ferromagnetic substance, all atoms within a region (called a magnetic domain) have their electron magnetic moments aligned the same way. In the absence of a magnetic field, the domains are randomly oriented. B. In a magnetic field, all domains (and all electron magnetic moments) align with the field. Because all moments align with the field, the substance is strongly attracted to a magnet. When the magnetic field is removed, the domains may remain aligned (resulting in a permanent magnet) or they may revert to the random orientation of domains.

When electrons are paired in an orbital, their opposite magnetic moments create opposite magnetic fields that cancel each other: substances with all electrons paired are neither paramagnetic nor ferromagnetic. They exhibit only diamagnetism and are said to be diamagnetic. For example, argon has a ground state electron configuration of 1s22s22p63s23p6, where each subshell is full and each orbital therein contains a pair of electrons with opposite spins. Hence, a sample of argon is diamagnetic.

Paramagnetic and diamagnetic materials do not act as permanent magnets. A magnetic field is required to align the magnetic moments and make them magnetic. The magnitude of paramagnetism can be measured by weighing a sample with a highly sensitive balance and then weighing it again with a strong magnet just below the sample. A sample that is attracted to the magnet appears heavier because the magnet attracts it downward. The increase in apparent weight is proportional to the number of unpaired electrons.

Exercise: Diamagnetic and Paramagnetic Ions

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Chemistry 109 Fall 2021 by John Moore, Jia Zhou, and Etienne Garand is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.