Why are the electronic configurations of Copper and Chromium exceptional?

[kay]

Why are the electronic configurations of Copper and Chromium exceptional? What is so different about them?

 

[Simon Bridge]

Depends what you mean by "exceptional".
Do you know what the electronic configurations are?
How do they compare to other elements (i.e. in the same group)?
Do copper and chromium have some special properties?

 

[kay]

They say that ' One might be puzzled by the fact that chromium and copper have 5 and 10 electrons in 3d orbitals rather than 4 and 9 as their position would have indicated with two electrons in the 4s orbital.'
Then they talked about the reason being half filled and completely filled subshells, which i could not understand at all.

 

[Simon Bridge]

Who is "they"?

 

[kay]

The authors to my book. :P

 

[Simon Bridge]

And who are these mysterious "authors to my book"? Which book? Is it a secret?

It sounds like "they" are telling you the rules for populating electron orbitals in the "orbital model" for atomic structure. In order to help you I need to know more about what it is that you could not understand. From what you've told me, all I can do is advise that you read another book on the same subject.

 

[Satvik Pandey]

Hi kay,
Half filled and full filled orbitals have extra stability due to greater exchange energy and symmetry.

 

[kay]

Yes satvik i do know that. :/
But the thing is that i am not able to understand the fact that HOW are the elec configurations different/exceptional?
And how does the half filled or completely filled state of orbitals influence the elec configs (in the case of chromium and copper).
Tell me this.

 

[Borek]

Just by following the Aufbau principle you should have for the consecutive elements (each one having one electron more than the previous one):

d³s²
d⁴s²
d⁵s²
d⁶s²

However, total energy is that of dⁿ plus that of s^m. Energy of d⁵ is lower than the energy of d⁴ by enough to make d⁵s¹ preferred over d⁴s². So, instead of following the Aufbau principle "blindly" ground states of the consecutive elements go like

d³s²
d⁵s¹
d⁵s²
d⁶s²

Actually from what I have read the common explanation about extra stability of half filled orbitals is not entirely true, but it is used in almost every book I can think of.

 

[Satvik Pandey]

Yes satvik i do know that. :/
But the thing is that i am not able to understand the fact that HOW are the elec configurations different/exceptional?
And how does the half filled or completely filled state of orbitals influence the elec configs (in the case of chromium and copper).
Tell me this.

According to Aufbau principal the outer most electronic configuration of Cr is 3d$^{4}$4s$^{2}$ but due to extra stability of half filled orbital the electronic configuration of Cr is 3d$^{5}$4s$^{1}$ as d$^{5}$configuration is more symmetrical than that of d$^{4}$ configuration and greater no of exchange is also possible in case of d$^{5}$ that is 10 where as in case of d$^{4}$ no of possible exchange is 6 hence the exchange energy is less. Therefore the ground state electronic configuration of Cr is 3d$^{5}$4s$^{1}$. it is just a qualitative explanation.