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[physicsss]

Last Minute Question--Test Hours away

So my book has the electric configuration of the period 4 transition elements say, for chromium, as, 3d5 4s2 because it says that once the d orbital has an electron, its energy is lowered. But i read everywhere else and my previous chem classes that the 3d orbitals have a higher energey that the 4s and that's why it is placed after after the 4s in the periodic table...can anyone clear this up for me?

Thanks a bunch!

 

[Gokul43201]

Cr actually has the configuration 4s1 3d5 (instead of 4s2 3d4) because half-filling subshells reduces the energy.

The 3d orbitals definitely have a higher energy than 4s. Whether you choose to write 4s1 3d5 or 3d5 4s1 is just a matter of convention. Either is acceptable (but not 4s2 3d5 for Cr).

 

[dextercioby]

I wouldn't say that.I've never seen anywhere 3d5 4s1,because the convention for writing down shells is pretty clear.It should indicate the correct order of energy increase of shells and subshells.What would happen if one wrote:
$$1s2 \ 2p6 \ 2s2 \ 3d10 \ 3s2 \ 3p6 ...$$ total chaos,huh...?

Daniel.

 

[GCT]

Well, I believe Cr has an interesting explantion to it (what Gokul said seems sound), I'll have to check up on it.

 

[Gokul43201]

I wouldn't say that.I've never seen anywhere 3d5 4s1,because the convention for writing down shells is pretty clear.It should indicate the correct order of energy increase of shells and subshells.What would happen if one wrote:
$$1s2 \ 2p6 \ 2s2 \ 3d10 \ 3s2 \ 3p6 ...$$ total chaos,huh...?

Daniel.

I've seen both ways be used. I can't agree, in general, with the sense of writing it as 3d5 4s1 but I know that it does sometimes get written that way. And the possibility of changing order comes up only in the sub-shells succeeding those of the closest smaller noble gas. eg : Cr = [Ar] 3d5 4s1 . In this case, however, it makes sense to write it this way, since $Cr^+$ = [Ar] 3d5 4s0.

 

[dextercioby]

I've said what I've been taught in (high) school.Plain simple.And i gave the reasons for which i think that my version is correct.The rest is just interpretation which I'm not caring to consider.

Daniel.