posted by Moi .
I have a question regarding the valence electrons of transition metals. I am aware of quantum numbers etc and how the s orbital gets filled before the d orbital but my question is that is if was asked to lets say group some elements according to their valence electrons,and among those elements there was lets say Zr, would I consider Zr to have only two valence electrons, or would I also count the electrons in the d sub shell.
Counting the outside electrons is the way we identify the valence electrons in main group elements; however, as you point out, this is not without its problem for transition elements. For those elements, we count the electrons outside the lower core inert element. Here is a site on wikipedia that explains exactly how this works. The example given is for Mn; therefore, instead of two for the 4s2 electrons, we count the two for the 4s2 as well as the 5 in the 3d5 orbitals to make a total of seven valence electrons.