Question

Why do we go down from 4s before 3d?

A) Decrease in principal quantum number

B) Increase in effective nuclear charge

C) Filling of inner shells

D) Stability of half-filled and fully-filled subshells

Answer 1

The 4s subshell is filled before the 3d subshell due to the stability of half-filled and fully-filled subshells; this shift occurs in elements like Cr and Cu.

Step-by-step explanation:

The reason we go down from the 4s subshell before the 3d subshell is due to the stability of half-filled and fully-filled subshells. In the case of Cr and Cu, the 3d subshell is shifted to gain the extra stability of a half-filled (Cr) or fully-filled (Cu) subshell. This stability outweighs the energy difference between the 3d and 4s subshells, causing the 4s subshell to be filled before the 3d subshell.