How do you find the number of valence electrons in a transition metal?

Answer 1

The concept of valence electrons although archaic is helpful in determining the 'valency' of an element. It works for the lighter main group elements but fails completely in the case of transition metals which have variable oxidation states. The modern concept of electronic config and the associated laws must be used.

Answer 2

Most of the transition metals can "change" their number of valence e- by moving the inner "d" levels electrons to the valence level. Some of them have several possible oxidation numbers, so the easiest way to figure out their charge is to see what anions they are bonded to.

FeCl2 Fe is bonded to 2 ions that have a 1- charge each, so the total negative charge is 2-. That means the total positive charge must be 2+.

CuSO4 Cu is bonded to one oxygen ion which is a 2- charge, so the total positive charge must be 2+.

Fe2O3 Fe is bonded to 3 oxygen ions which are a 2- charge, so the total negative charge is 6-. That means the total positive charge must be 6+. Since two Fe give a 6+ charge, then one Fe must have a charge of 3+.