Greetings, I have some questions regarding electronic configuration. 1. In an io

Question

Greetings,

I have some questions regarding electronic configuration.

1. In an ion Fe3+ the electron config was shown as: [Ar]3D5.

^^ I am confused to why 4S2 was skipped? I know that 4Sfills before 3D due to energy conservation. And electrons removefrom the highest shell, so would it remove from 3D, since it hashigher energy? In this case, iron has lost 3 electrons, so it'satomic number is now 23. Not sure why 4S was skipped. May someonesimplify this concept?

2. In another example, it stated that Br- configurationis: [Ar] 4S23d104p6 or [Kr]

^^ Yet, here 4S2 was used. I understand that Br gained an electron,so it's atomic number is 36. Not sure why 4S2 was used here and notin example 1?

3. Finally, there was another example that stated that Be with anexcited electron configuration becomes1S22S12P1

^^ I am confused on why this config is this way? I know thatelectrons want to be at the lowest energy level possible.


Easy explanations to these questions would be greatly appreciatedand I will rate a lifesaver to the person who can explain theseconcepts in the easiest way. Analogies may help.

Explanation / Answer

1) The electron configuration of the atom Fe is[Ar]4s23d6.

The ion Fe2+ loses two electrons from thehighest n value of 4, so

Fe2+ = [Ar]3d6

The ion Fe3+ loses three electrons, the two from the4s orbital and one more, so

Fe3+ = [Ar]3d5

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