When a transition metal forms a positive ion, the electrons are lost first from

Explanation

Although 4s fills before 3d, once the d subshell is occupied the 4s level becomes the higher in energy, so it is emptied first on ionisation, and the Fe2+ ion is therefore 3d6 rather than 3d4 4s2. This detail is asked frequently because it appears to contradict the filling order. It explains why almost every transition metal shows a plus 2 oxidation state.