Why is the electron configuration of chromium?

1 Answer

Answer:

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.