[NewtoAtoms]

Hello everyone,

I am new to chemistry so please pardon my naive questions!

If I was asked to give the electronic configurations of Cr, I would first note that Z=24, and that n=4 from the periodic table.  HOWEVER, if I plot out the electrons using energy level diagrams, it would be as follows

3d   6 to give me a total of 24 electrons
3p   6   
3s   2
2p   6
2s   2
1s   2
1s22s22p63s23p63d6

But I  know from the periodic table that n=4, which means there are some electrons in the n=4. 
When I cheat and look on the internet I see that the electron configeration is 1s22s22p63s23p63d54s1.

can anyone explain to me why or how we got to this??  Why does this happen and how do I know when to look for this in other elements?

Thank you for you help.

[tamim83]

It is one of the common anomolies in the periodic table.  Half filled subshells are quite stable so instead of Cr having a 3d⁴4s² configuration, it has a 3d⁵4s¹ configuration, which is more stable.  I imagine that this has been confirmed spectroscopically.  It happens with Mo, W, and Sg as well since they live in the same group. 

Also, full shells are very stable because they have a "psuedo" noble gas configuration.  So Cu, Ag, and Au have a nd¹⁰ms¹ configuration instead of a nd⁹ms¹ configuration. 

[NewtoAtoms]

Thank you!! 

Of course I can always find an anomolie, should there be one to find! 

Just to practice what I learned I went on to plot the electron configuration of more electrons to see if I could understand it. However I was shocked to see that:

Cr has 3d5 (which is not complete but sent only 1 electron to 4s) and 4s1

BUT

Ti has 3d2 (which is also not complete but it sent 2 electrons to 4s) but 4s2

Why didn't Cr complete it's 4s2, like Ti did?
Why didn't Cr go 3d4 and 4s2 similar to Ti.

Would you suggest that the simplest way is to just memorize these bizzare occurances with electron configurations?

Thank you for all your *delete me*

[Yggdrasil]

The 4s orbital is lower in energy than the 3d orbitals, so it will fill up before the 3d orbitals.  The difference in energy between the 4s and 3d orbitals, however, which will be important to take into account later.  Therefore, the order of filling orbitals is:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

Based on this principle, we would expect Cr to have the electron configuration [Ar]4s²3d⁴.  However, shells that are exactly half filled are more stable.  Therefore, even though it seems like it would cost energy to promote a 4s electron to a 3d orbital, the chromium atom gains more stability from having a half filled set of d orbitals than the stability it loses from promoting an electron from the 4s to a 3d orbital.  A similar phenomenon is seen with copper.  Copper has an electron of [Ar]4s¹3d¹⁰ instead of [Ar]4s²3d⁹ because the gain in stability from filling the d orbitals makes up for the energy penalty of promoting a 4s electron to a 3d orbital.