When do electrons get shared between orbitals
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When do electrons get shared between orbitals

[From: ] [author: ] [Date: 12-11-10] [Hit: ]
3d doesnt start until Sc.So by its place on the periodic table Cr should be 4s2 3d4.The second thing is that the s orbital has 2 electrons and they repel each other.But theres also an empty d orbital (there are 5).So one of the electrons in the s orbital can be promoted to the empty 3d orbital making the configuration 4s1 3d5.This half-filled arrangement is a lower energy arrangement.......
The question is.. "Select ground state electron configuration of Cr."

The answer says its [Ar]4s^13d^5 but im wondering why its not [Ar] 3d^6?

Im assuming an electron gets shared to fill the d orbital, but when do you know to do that?

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You're looking at two quirky things.
First, it's the initial instance of an energy sublevel of higher n (4s) preceding a sublevel of lower n (3d).
K and Ca have outer electrons in the 4s level.
3d doesn't start until Sc.
So by its place on the periodic table Cr should be 4s2 3d4.

The second thing is that the s orbital has 2 electrons and they repel each other.
But there's also an empty d orbital (there are 5).
So one of the electrons in the s orbital can be "promoted" to the empty 3d orbital making the configuration 4s1 3d5.

This half-filled arrangement is a lower energy arrangement. and along with a completely filled sublevel is said to be "especially stable."

The explanation is a bit contrived, but the logic should work for you.

A similar thing seems to happen with C also: s2p2 → s1p3 which helps explain why C typically forms 4 bonds.
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