In Fe atom, which subshell experience the lowest effective nuclear charge and are hence the first removed upon ionization?

(A) 1s

(B) 3p

(C) 3d

(D) 4s

(D) 4s

Explanation:

The electronic configuration of a Fe atom is:

1s² 2s² 2p⁶ 3s² 3p6 3d⁶ 4s².

The subshell located farthest from the nucleus is 4s, that's why it experiences the lowest effective nuclear charge.

This can be seen when looking at the electronic configurations of Fe⁺² and Fe⁺³:

Fe⁺² ⇒ 1s² 2s² 2p⁶ 3s² 3p6 3d⁶.

Fe⁺³ ⇒ 1s² 2s² 2p⁶ 3s² 3p6 3d⁵.