Hi,

The electron configuration of Fe is [Ar]4s2 3d6. The 4s electrons are lost before the 3d electrons are lost. Reason: All orbitals shrink when the atom becomes a cation; electron-electron repulsion would soar in a shrunken 4s orbital. The electron configuration of Fe2+ is [Ar] 3d6. The next electron is removed from the 3d subshell. The electron configuration of Fe3+ is [Ar] 3d5. The 3d subshell is now half-filled, with one electron in each of the five 3d orbitals, a very stable configuration.

1s2 2s2 2p6 3s2 3p6 3d5 4s1

Hope this helps.