Magnetism

Paramagnetism arises from the presence of unpaired electrons. Transition elements have their valence electrons in the and subshells. Because the -subshell can accommodate ten electrons and is partially filled across the series, most transition-metal atoms and many of their ions retain one or more unpaired -electrons.
These unpaired electrons give a net magnetic moment ( , where is the number of unpaired electrons), hence paramagnetism.
Further, transition metals exhibit variable oxidation states; during oxidation or complex formation, electrons are removed from and levels, often leaving unpaired -electrons in the cations/complexes.
Notes/Exceptions: Closed-shell cases like , , are diamagnetic. Some strong-field, low-spin complexes can also become diamagnetic if all electrons pair.