What kind of electronic configurations are possible?

Question

Let us take $d^6$ orbital as an example:

A) ↑ | ↑ | ↑ ↓ | ↑ | ↑ |

Now , this one 1 way. My Q is that how is it possible that in the 3rd box , we have a paired electron but not in the 1st one. Like it should pair in a series right ? From 1st , 2nd & then 3rd one.

B) We know ↑ ↓ | ↑ | ↑ | ↑ | ↑ |

This one I know is correct as all the examples in my textbook are written in this order but in a Q , Option A is also correct.

I want to know why & how ?

Answer 1

This misconception arrises because most of the textbooks and websites starts pairing electrons from left to right in a series. But in general we can pair an electron in any of the degenerate orbitals if all degenerate orbitals are filled with an electron having same spin either up or down ( which is called Hunds rule).

• Note: The orbitals which are having same energies are called degenerate orbitals.

In an atom all five d-orbitals having same principle quantum number are degenerate orbitals. As all are of equal energies you can pair an electron in any of the five d-orbitals you like.

• This type of degeneracy is lost when it is attacked by strong field ligands. This is called crytal field splitting of orbitals. (which is out scope of this question)