unpaired electrons
Summary
Unpaired electrons are electrons alone in an orbital, so their spin magnetic moments are not canceled by a partner. Atoms with them are attracted to magnetic fields (paramagnetic, or ferromagnetic in domain-forming metals like iron); fully paired atoms are diamagnetic and weakly repelled.
Detail
Meaning: Each electron has spin, which gives it a tiny magnetic moment. Two electrons in one orbital must have opposite spins (Pauli exclusion principle), so their moments cancel. An unpaired electron is alone in its orbital, so its moment is not canceled and the atom has a net magnetic moment. Hund's rule says degenerate orbitals fill singly before pairing, which is how you count them. In organic chemistry, a radical is a species with an unpaired electron.
On the MCAT: Write the electron configuration of an atom or ion, count the unpaired electrons, and predict magnetic behavior. None unpaired: diamagnetic, weakly repelled by an external field. One or more unpaired: paramagnetic, weakly attracted; without a field the moments point randomly, so no magnetization remains. In Fe, Co and Ni, unpaired spins align within domains, giving ferromagnetism: strong attraction and possible permanent magnetization. Example: Fe³⁺ (3d⁵) has five unpaired electrons; Zn²⁺ (3d¹⁰) has none.
Common confusion: Having unpaired electrons does not make a material a permanent magnet. Most paramagnetic substances show no magnetism without an external field; only ferromagnetic materials keep alignment. For transition-metal cations, remove the 4s electrons before the 3d electrons, then count what is left.
Sources
- University Physics
- Physics
- Fundamentals of Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.