Half-Filled Shells and Stability explanation [duplicate]








**This question already has answers here**:
 
 




[Why do elements in columns 6 and 11 assume 'abnormal' electron configurations?](/questions/151/why-do-elements-in-columns-6-and-11-assume-abnormal-electron-configurations)

 (4 answers)
 

Closed 5 years ago.



I am reading a book about Advanced Chemistry, and it is discussing the subject of half-filled orbitals. 


The book notes that Chromium has an electron structure of $1s^2 2s^2 2p^63s^23p^63d^54s^1$



> 
> There is also some added stability when a sub-shell is half-full as this minimises the mutual repulsion between pairs of electrons in full orbitals. 
> 
> 
> 


Now, I'm confused by this explanation. 


Is it correct to conclude that if a sub-shell is half filled, the repulsion **between electrons in the half-filled subshell and other subshells with fully-filled orbitals** is reduced (e.g. between the $3d$ and $3p$ subshell in Chromium)?




I actually just had this in class, and it confused me as well. This site helped me to understand it a bit better: 
<http://www.chemguide.co.uk/atoms/properties/3d4sproblem.html>


The author, a retired chemistry professor, states:



> 
> "Many chemistry textbooks and teachers try to explain this by saying that the half-filled orbitals minimise repulsions, but that is a flawed, incomplete argument. You aren't taking into account the size of the energy gap between the lower energy 3d orbitals and the higher energy 4s orbital."
> 
> 
> 


 0