The electron configuration of "Co"^(3+) is ["Ar"] 4s 3d^5.
"Co" is in Period 4 of the Periodic Table, and "Ar" is the preceding noble gas.
Cobalt is also in Group 9, so it must have 9 valence electrons.
The valence shell configuration is therefore 4s^2 3d^7, and the core notation is
bb"Co": ["Ar"] 4s^2 3d^7
When a transition metal forms an ion, the s electrons are removed before the d electrons.
We would predict the electron configuration of "Co"^(3+) to be
bb"Co"^bb(3+): ["Ar"] 3d^6.
The 4s and 3d sublevels are nearly identical in energy, so the ion can become more stable by moving one of the 3d electrons to the 4s level.
Then, both the 4s and 3d levels are half-filled, and the ion gains a little more stability.
The electron configuration becomes:
bb"Co"^bb(3+): ["Ar"] 4s 3d^5
