The difference between magnetism and chemical affinity.


Since hemoglobin is/contains iron, is its affinity for oxygen synonymous with ferrous magnetism? What are the significant differences at the molecular level between affinity and magnetism?

In: Chemistry

Magnetism is more restricted to an attraction between positive and negative forces. Chemical affinity is less of a give and take scenario and can involve more of a sharing-type approach.

To address this question, we have to clear up a misconception you may have about hemoglobin and heme. Heme is a molecule that is able to covalently bind oxygen atoms, meaning that the resulting molecule has oxygen completely incorporated into it. The binding of oxygen to heme has nothing to do with the magnetic properties of iron, but the size and electron density of the iron atom itself.

Be sure not to conflate different uses of “affinity” as well. Electron affinity, chemical affinity, and affinity in the biological sense (i.e. ligand-binding) are all related, but very different things.

The similarities between affinity and magnetism are really only that both are based in electronic properties (inasmuch as chemical bonds are electronic). Affinity is a property of two or more specific compounds, measuring how often and how strongly they interact with each other, and can be applied to any interaction. Magnetism is a physical attribute of individual atoms, particles, and materials; causing objects to be attracted or repelled from other objects.

(See also: the difference between chemical and physical properties.)

Since everything is made up of atoms, everything is magnetic to some degree, but you can only talk about affinity if you’re comparing two things. Magnetic force can increase or decrease a compound’s affinity to another compound, but it isn’t the only factor at play.