Question

Electronic configuration of fe2+,fe3+,Cu,cr

Answer 1

fe+2  [ 18Ar ] 3d6
fe+3 [ 18Ar ] 3d5
cu [ 18Ar ] 4s1 3d10
cr [ 18Ar ] 4s1 3d5

Answer 2

Fe - 26 - [Ar] 4s² 3d^6 
Fe+2 -> Fe - 2e- ---> [Ar] 3d^6 ( since, before removing electrons from 3d, ions should be removed from 4s)
Fe+3 -> Fe - 3e- ---> [Ar] 3d^5( since, before removing electrons from 3d, ions should be removed from 4s and then 3d)
Cu - 29 - [Ar] 4s^1 3d^10 ( it is an exceptional case)
Cr - 24 - [Ar] 4s^1 3d^5 (it is an exceptional case) 
electronic configuration of Ar - 1s² 2s² 2p^6 3s² 3p^6