Answer:
the 4s subshell is lower in energy than the 3d subshell
Explanation:
In the element number 24 which is chromium
the electronic configuration should be according to Aufbau principle
will be = [tex]1s^22s^22p^63s^23p^64s^23d^4[/tex]
but in reality it is = [tex]1s^22s^22p^63s^23p^64s^13d^5[/tex]
The 6 electrons in the outermost shell will be divided as 5 in 3d subshell and 1 in 4s shell, because half filled orbital [tex]4s^13d^5[/tex] will be more stable than [tex]4s^23d^4[/tex].
