[shehri]

Hi all,
While distributing electrons in transitioal elements,it's observed that sometimes electrons in 's' & 'd' orbitals of the valence orbit're kept incomplete.Normally we fill second most orbit (according to n+l rule) first & then fill last orbital but  in transitional elements  both i.e s & d orbitals remain  unfilled.Why?Plz. clear me about that.Thanks.

[FeLiXe]

you have to remember that the n s and the (n-1) d orbitals are very close in energy. sometimes s is lower, sometimes d is lower, sometimes they will act like one shell that is filled according to Hund's rule.

I don't think you can predict the behaviour without actual quantum mechanics. effective charge according to the slater rules may explain some phenomena

[Yggdrasil]

There is a special stability associated with a completely full or half full set of orbitals, which can often override the conventional rules of filling orbitals.  For example, Cr is 4s¹3d⁵ and Cu is 4s¹3d¹⁰ for this reason.