Iron has the ground state configuration [Ar]3d6 4s2. Which electrons leave first when the Fe2+ ion forms?

1. The d orbitals of a transition element fill last when the configuration is built up. 2. Ionization does not simply run that filling order backwards. 3. For a transition metal the outermost s electrons are removed first. 4. In iron the outermost s subshell is 4s, holding two electrons. 5. Removing both 4s electrons gives Fe2+ with the configuration [Ar]3d6. 6. Taking the two 3d electrons instead is the classic trap of reversing the filling order. 7. Taking one 4s and one 3d would give a mixed removal that does not match the observed ion. 8. The 3p electrons sit in a filled inner shell and are far too tightly held to leave. _Source: OpenStax Chemistry (CC BY 4.0), Chs 7 and 17, section 7.1 Ionic Bonding_