Transition metals owe their location in the periodic table to the filling of the d subshells. When these metals are oxidized, however, they lose their outer s electrons before they lose electrons from the d subshell. For example, the electron configuration of Fe is [Ar]4s²3d⁶, whereas that of Fe²⁺ is [Ar]3d⁶. Formation of Fe³⁺ requires loss of one 3d electron, giving [Ar]3d⁵. Most transition-metal ions contain partially occupied d subshells.