diamagnetism and paramagnetism

Hold a substance near a strong magnet. Most everyday things are faintly pushed away; a few are weakly pulled in. This split in behavior, invisible to the eye but easy to measure on a balance, turns out to be a direct window into how a substance's electrons are paired up — which is why magnetism is one of the inorganic chemist's most powerful diagnostic tools.

The two behaviors come from electron spin. Every electron is a tiny magnet, but when two electrons share an orbital their spins point opposite ways and cancel. A substance whose electrons are all paired has no net magnetism of its own; placed in a field it is very weakly repelled, and we call it diamagnetic (all substances have this small repulsion, but it dominates only when nothing else competes). A substance with one or more unpaired electrons has a real net magnetic moment from those lone spins; placed in a field the moments line up with it and the substance is pulled in — this stronger attraction is paramagnetism, and it is typically far larger than the diamagnetic background.

For transition-metal complexes the link is beautifully direct: paramagnetism means unpaired electrons, diamagnetism means none, and the strength of the paramagnetism counts how many unpaired electrons there are. Measuring it therefore tells you the d-electron configuration at a glance, which is how chemists distinguish high-spin from low-spin, infer the oxidation state and geometry, and check a bonding model against reality. The honest caveat: paramagnetism here describes independent, non-interacting moments; when the moments in a solid start talking to each other you get the cooperative behaviors of ferromagnetism and antiferromagnetism, which are a different and richer story.