Electronic Configuration Question

[relativitydude]

How come the valance electrons of MO is [KR]5s^1 4d^5 and not

[KR]5s^2 4d^4

[Sirus]

Because of the extra stability associated with a half-filled d-orbital. See here (http://itl.chem.ufl.edu/2045/lectures/lec_11.html).

Scroll down to about half way where you see 'exchange' in blue; the configuration charts below that should be helpful as well.

PS: Be careful with your notation; the elements MO and KR do not exist, and should be written as Mo and Kr to denote the elements you speak of.

[relativitydude]

Thank you, Sirus.

That makes sense, but how would I predict such a thing?

[poolwin2001]

All half filled and full filled orbitals have extra stability.So much that the electron from s orbital may go to the inner orbital.check out e config of Cr,Cu etc.