Free radicals are atoms or molecules containing unpaired electrons. Free radicals are very reactive atoms or molecules that have unpaired electrons. Molecular compounds that contain one or more unpaired electrons are paramagnetic. Compounds are diamagnetic when they contain no unpaired electrons. It is paramagnetic with one unpaired electron per molecule. Metal complexes that have unpaired electrons are magnetic. Some molecules contain one or more unpaired electrons, creating radicals. All the trivalent lanthanide ions, except lanthanum and lutetium, have unpaired f electrons. For molecules it signifies that there are unpaired electrons. In contrast, O is paramagnetic, containing two unpaired electrons. The lowest-energy state with maximum multiplicity usually has unpaired electrons all with parallel spin. Free radicals are oxidizing agents that have one unpaired electron, making them highly reactive. Transition metal compounds are paramagnetic when they have one or more unpaired "d" electrons. However the ground state of this molecule is paramagnetic, indicating the presence of unpaired electrons. A ferromagnet, like a paramagnetic substance, has unpaired electrons.