Why is the electron configuration of chromium?

1 Answer
Dec 14, 2015

It is energetically favored.

Explanation:

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

1s22s22p63s23p24s23d4

Or, condensed using noble gases:

[Ar]3d44s2

However, the actual configuration of Chromium is:

[Ar]3d54s1

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]4d55s1

Copper:

[Ar]3d104s1

Silver:

[Kr]4d105s1

And gold:

[Xe]4f145d106s1

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