One Ni atom has 28 electrons and its electronic configuration is : [Ar] 4s2 3d8
It becomes Ni2+ by losing 2 electrons, hence configuration of Ni2+ is : [Ar] 4s0 3d8
So, nickel loses two electrons from the 4s orbital not the 3d orbital.
• This is due to the two contrary facts: as 4s has a lower energy level than 3d orbital (Aufbau principle) then it should have been the 3d electrons that should get lost.
• But, as the 4s orbital is further from the nucleus and is shielded by the 3d electrons it is easier to lost 4s electrons than 3d electrons. Moreover, it leaves the 3 d electrons as valence shell electrons which is more stable than losing them.
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