Question

During formation of cobalt 3+ ion why electron is removed from 4s first

Answer 1

When d-block elements form ions, the 4s electrons are lost first. The 4s electrons are lost first followed by one of the 3d electrons. This last bit about the formation of the ions is clearly unsatisfactory. We say that the 4s orbitals have a lower energy than the 3d, and so the 4s orbitals are filled first.
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