CHEMISTRY COMMUNITY

During lecture on Friday, we went over exceptions for the order of subshells for Chromium and Copper. What reason is behind the formation of the subshells? Is it for the purpose of making each of the subshells more stable?

Yes, electrons will always act in a way that yields the most stability possible. Drawing out the version you would expect one of them to have vs. the actual version will help you conceptualize more!

"Exceptions are based on the fact that half-full or full shells or subshells are more stable than partially filled ones. When the difference in energy levels between two subshells is small, an electron may transfer to the higher level shell to fill or half-fill it. The electron occupies the higher energy level shell in violation of the Aufbau principle because the atom is more stable that way."

https://sciencing.com/what-is-the-basis ... 10232.html

This source explains the reasoning well.

For chromium, the electron configuration would have looked like the following if it followed "normal" conventions: [Ar]4s²3d⁴
And for copper: [Ar]4s²3d⁹
However, the atom finds more stability when the d-subshells are half-filled or fully filled:
Cr: [Ar]4s¹3d⁵
Cu: [Ar]4s¹3d¹⁰

The rest of the elements in those columns work much the same way.

In one of the UA sessions that I went to today, the UA told us to write Cr as [Ar]3d⁵4s¹ instead of [Ar]4s¹3d⁵. I do not know why he said that but can someone better explain this to me?

I think that it depends on the conditions at which the most stability is achieved.

DesireBrown3K wrote:In one of the UA sessions that I went to today, the UA told us to write Cr as [Ar]3d⁵4s¹ instead of [Ar]4s¹3d⁵. I do not know why he said that but can someone better explain this to me?

As the 3d orbital gains more electrons, it ends up having less energy than the 4s orbital. Since we're supposed to write electron configurations in such a way that we go from lower to higher energy levels, 3d is displayed before 4s.