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 1st and 4s after it ???
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 4s1 Configuration in order to attain symmetry.