Question

Why is the electron configuration of chromium?

Answer 1

It is energetically favored.

Explanation:

Looking at the periodic table, you would expect Chromium to have this configuration:

`1s^2 2s^2 2p^6 3s^2 3p^2 4s^2 3d^4`

Or, condensed using noble gases:

`["Ar"] 3d^4 4s^2`

However, the actual configuration of Chromium is:

`["Ar"] 3d^5 4s^1`

So why is this? It has to do with main the arrangement of sublevels in the electron cloud. Even though `4s` belongs to a higher main energy level than `3d`, the energy levels are similar. This actually allows electrons to move between the sublevels if it is energetically favorable.

This happens in Chromium, as one `4s` electron moves to the `3d` sublevel. Why? There are two main reasons:

1. The `3d` orbital is slightly lower in energy, and minimizing repulsions in the `4s` orbital by moving one of the `4s` electrons to a close-lying `3d` orbital minimizes the ground-state energy of chromium.
2. Hund's Rule: It is energetically favorable to maximize the spin state in a sublevel. Since two opposite spins result in a total spin of `0`, maximizing this tends to require as many electrons in of same spin in different orbitals as possible. So, in this case, an electron moves to `3d` and is unpaired, therefore maximizing the spin state.

You will see a similar situation where an `s` electron moves to a `d` sublevel with Molybdenum:

`["Kr"] 4d^5 5s^1`

Copper:

`["Ar"] 3d^10 4s^1`

Silver:

`["Kr"] 4d^10 5s^1`

And gold:

`["Xe"] 4f^14 5d^10 6s^1`

In all of these scenarios, the new redistribution of the electron is energetically favorable compared to the predicted configuration.