Q.State the defining structural feature of a transition element. Using this definition, explain why copper (configuration $[\text{Ar}]3d^{10}4s^1$ in the free atom) is still classified as a transition element.

The definition of a transition element does not require the free atom to have a partially filled d subshell -- it requires that the atom or at least one of its commonly formed ions does.

Copper's free-atom configuration, as established earlier in this chapter, is the anomalous $[\text{Ar}]3d^{10}4s^1$, in which the d subshell is already completely filled ($d^{10}$). Taken on its own, this might seem to place copper in the same category as zinc, cadmium and mercury, which are excluded from the transition elements precisely because their d subshell is filled in every state they commonly adopt.

The crucial difference is what happens on ionization. Copper's most common oxidation state in its compounds is $+2$ (as in $\text{CuSO}_4$, $\text{CuO}$, $\text{CuCl}_2$), formed by losing the single $4s$ electron together with one $3d$ electron: $\text{Cu} \rightarrow \text{Cu}^{2+} + 2e^-$ gives the configuration $[\text{Ar}]3d^9$ -- a genuinely partially filled d subshell, with one vacancy. Because this commonly formed ion, $\text{Cu}^{2+}$, has a partially filled d subshell (even though the neutral atom does not), copper satisfies the "ion" clause of the definition and is correctly classified as a transition element.

This is exactly why $\text{Cu}^{2+}$ compounds are strongly coloured (blue, by the same d-d-transition reasoning developed elsewhere in this chapter) and why copper shows genuinely variable oxidation states ($+1$ as $\text{Cu}^+$, $3d^{10}$, and $+2$ as $\text{Cu}^{2+}$, $3d^9$) -- both hallmarks of true transition-metal behaviour that zinc, cadmium and mercury do not show, since none of their commonly formed ions ever departs from $d^{10}$.

State the full definition (atom OR ion), work out Cu2+'s configuration (3d9) from the free atom's 3d10 4s1, and show it is partially filled even though the free atom is not.

Checking only the free atom's configuration and concluding Cu is not a transition element (this misses the 'or ion' half of the definition, which is exactly the point of this question).