Question

The subshell 4s has the smaller energy than the subshell 3d.But the subshell configuration of Cr is 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹.why did 3d came 1^st and 4s after it ???

Answer 1

symmetry leads to stability.
Thus, the electronic configuration in which all the orbitals of the same sub-shell are either completely filled or exactly half filled are more stable because of symmetrical distribution of electrons.

Cr has 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s^{1
Configuration​​​​​ in order to attain symmetry.}