Why are transition metals said to have different valencies? I understand that the term valency refers to the number of bonds an element would be able to form with either chlorine or hydrogen. If this is directly related to the number of valence electrons an atom has, why do transition metals have variable valencies?
I think it's kind of a poor use of terms here. When talking about transition metals, the valency generally refers to the oxidation state of the metal atom. E.g: hexavalent chromium is $\ce{Cr^6+}$, not necessarily a chromium bonded to six other things.