An element in which electrons partially fill the d or f subshell in any common oxidation state, so that it can participate in chemical bonds in two shells rather than just one. A transition element is a hard metal with high melting and boiling points that is a good conductor of heat and electricity, has several oxidation states, probably forms coloured compounds, is likely to be paramagnetic (its atoms and molecules can be aligned in the direction of an applied magnetic field) owing to its unpaired electrons, and may be a good catalyst.