# Which electron in cobalt is easiest to remove first?

Jun 3, 2016

It is the first $4 s$ electron chosen.

In cobalt, the higher-energy orbital when comparing the $4 s$ and the $3 d$ subshells is the $\setminus m a t h b f \left(4 s\right)$ orbital. In fact, the $4 s$ orbital is about $\text{4.02 eV}$ higher in energy.

The first electron removed from the $4 s$ orbital, which is doubly-occupied, is easy to remove because it is paired.

The electron pairing adds charge repulsion between the electrons, increasing the ease of removing the first one.

The second one in the $4 s$ orbital (after removing the first one already) is a bit harder to remove because it doesn't have the repulsion energy that it did when the electrons were paired.

Consider the ionization energies of the first two electrons:

First ionization energy: "7.881 eV" ("760.41 kJ/mol") (removing first electron)
Second ionization energy: "17.08 eV" ("1648.27 kJ/mol") (removing BOTH electrons)

Thus, the ionization energy for removing the second electron is $17.08 - 7.881 = \text{9.20 eV}$, which is "1.32 eV" ("127.36 kJ/mol") larger, and confirms that repulsion energy plays a role in the ease of removing the first electron relative to the second.