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Iron has the ground state configuration [Ar]3d6 4s2. Which electrons leave first when the Fe2+ ion forms?
AThe two 3d electrons
BOne 4s and one 3d
CThe two 4s electrons
DTwo of the 3p electrons
Answer & Solution
Correct answer: C. The two 4s electrons
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_
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