Metal atom with atomic number 21 has the differentiating electron enetering in 3d but not in 4s. However, in the formation of divalent cation 4s electrons are removed. Why?

Metal atom with atomic number 21 has the differentiating electron enet

1.	Metal atom with atomic number 21 has the differentiating electron enetering in 3d but not in 4s. However, in the formation of divalent cation 4s electrons are removed. Why?
Answer» Solution :While filling ELECTRONS 4s is filled FIRST and then 3D, based on Auf-bau PRINCIPLE. During the FORMATION of divalent cation, it is easy to remove lectrosn from 4s compared to that of 3d.