Question

Why are $Cu,Ag$ and $Ag$ included under transition elements even though they contain completely filled $d-$orbitals in their ground state?

Answer 1

These metals in their common oxidation states have incompletely filled d-orbitals. e.g, ${Cu}^{2+}$ has ${3d}^{10}$ and ${Au}^{3+}$ has ${5d}^8$ configuration.

$Cu$ displays two oxidation states $(+1$, $+2)$. In the +1 oxidation state, an electron is removed from the S-orbital. However, in the +2 oxidation state, an electron is removed from the d-orbital now becomes incomplete $\left({3d}^9\right)$. Hence, it is a transition element.