# What is the electron configuration of "Co"^"3+"?

Jan 3, 2016

The electron configuration of ${\text{Co}}^{3 +}$ is $\left[\text{Ar}\right] 4 s 3 {d}^{5}$.

$\text{Co}$ is in Period 4 of the Periodic Table, and $\text{Ar}$ is the preceding noble gas.

Cobalt is also in Group 9, so it must have 9 valence electrons.

The valence shell configuration is therefore $4 {s}^{2} 3 {d}^{7}$, 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 ${\text{Co}}^{3 +}$ to be

bb"Co"^bb(3+): ["Ar"] 3d^6.

The $4 s$ and $3 d$ sublevels are nearly identical in energy, so the ion can become more stable by moving one of the $3 d$ electrons to the $4 s$ level.

Then, both the $4 s$ and $3 d$ levels are half-filled, and the ion gains a little more stability.

The electron configuration becomes:

bb"Co"^bb(3+): ["Ar"] 4s 3d^5