diamagnetic materials

Meaning: In a diamagnetic material the atoms or ions have no net permanent magnetic moment, typically because every electron is paired and the spins cancel. An applied field still alters the electrons' motion and induces a weak magnetic moment opposite to the field, in the spirit of Lenz's law. The material is therefore pushed slightly away from the stronger field. The effect vanishes when the field is removed. Examples include water, most organic compounds, copper, graphite, bismuth and ions with filled subshells such as Zn²⁺.

Equation: M = χH, where M is the magnetization (A/m), H is the applied magnetic field strength (A/m), and χ is the magnetic susceptibility (dimensionless). For diamagnetic materials χ is small and negative; for paramagnetic materials it is small and positive.

On the MCAT: A passage gives electron configurations and asks which species is attracted to a magnet. Paired electrons only means diamagnetic and weakly repelled; one or more unpaired electrons means paramagnetic and weakly attracted. Ferromagnets (Fe, Co, Ni) are strongly attracted and can stay magnetized because of aligned domains.

Common confusion: Diamagnetic does not mean nonmagnetic. These materials do respond to a field, but by opposing it, and weakly. Also, diamagnetism is present in every material. In paramagnetic and ferromagnetic materials it is simply overwhelmed by the alignment of unpaired electron spins.