Question

What is the electron configuration of Co^2+ ?

Answer 1

Electronic configuration of Co is as follows: 1s² 2s²2p⁶3s²3p⁶4s²3d⁷

When Cobalt loses 2 electrons to become Co²⁺ it loses the electrons which are in 4s², not the ones in 3d⁷ because the electrons in 4s² have a high reactivity.

So Co²⁺still contains 7 electrons in it's valence shell. The new configuration of Co²⁺ is 1s² 2s²2p⁶3s²3p⁶3d⁷

explaination

The n+l rule tells you the order in which atomic orbitals are filled, and according to the rule the 4s orbital is occupied before the 3d orbital because it has lower energy. Thus, the electron configuration of Mn is [Ar]3d54s2 while that of Co is [Ar]3d74s2 But, the n+l rule, as many other rules of old quantum theory, is not 100% working, and thus, sometimes gives wrong electron configuration. For instance, in general, it is applicable only for neutral atoms in their ground state, and thus, if you apply it to cation Co2+Co2+ you will get the wrong electron configuration [₁₈{Ar}] 3d54s2.

As it is sometimes explained, the statement that 4s orbital is lower in energy than 3d orbital is true only when the orbitals are unoccupied. But while you fill 3d orbital with electrons it becomes lower and lower in energy and eventually ends up lower in energy than the 4s orbital. Thus, when electrons are lost from Co atom, they are lost from the 4s orbital first because it is actually higher in energy when both 3d and 4s are filled with electrons.