Cu, Ag & Au are regarded as transition elements. Why?

Explanation

In spite of having completely filled d-orbitals `(d^(10))` copperr, silve and gold are considered as transition elements because of least in one stable oxidation state of the atoms of these elements. D-orbital is partially filled with electrons. For example, the electronic configuration of `Cu^(2+)` is `1s^(2)2s^(2)2p^(6)3s^(2)3p^(6)3d^(9)4s^(0)`.