Which electron in cobalt is easiest to remove first?

It is the first `4s` electron chosen.

In cobalt, the higher-energy orbital when comparing the `4s` and the `3d` subshells is the `\mathbf(4s)` orbital. In fact, the `4s` orbital is about `"4.02 eV"` higher in energy.

![http://chem406.wikispaces.com/]()

The first electron removed from the `4s` 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 `4s` 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 = "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.