paramagnetic

Meaning: A paramagnetic species has one or more unpaired electrons. Each unpaired electron has a spin magnetic moment, and in an external field these moments tend to align with it, so the sample is weakly attracted into the field. The effect fades when the field is removed. A diamagnetic species has its electrons in pairs, so the spins cancel and it is weakly repelled. More unpaired electrons usually gives a stronger effect.

On the MCAT: Expect to write an electron configuration, apply Hund's rule, and count unpaired electrons. Fe³⁺ ([Ar] 3d⁵, five unpaired) is paramagnetic; Zn²⁺ (3d¹⁰) and Ca ([Ar] 4s²) are diamagnetic. For transition metal cations, remove 4s electrons before 3d. Passages may also describe a sample pulled toward a magnet and ask what that implies about its electrons.

Common confusion: The Lewis structure of O₂ shows paired electrons, so students call it diamagnetic. Molecular orbital theory places two unpaired electrons in the degenerate π* orbitals, and liquid O₂ is attracted to a magnet, so O₂ is paramagnetic. Also, paramagnetic is not the same as ferromagnetic: iron metal can keep strong magnetism, while paramagnetic samples typically do not once the field is removed.