Transition Elements: Question 1

Worked solution

Step 1: Work out the electron configurations involved

Zinc, $Z=30$, has the ground-state atomic configuration: $\text{Zn}:\ [\text{Ar}]3d^{10}4s^2$

Zinc has only one common ion, $\text{Zn}^{2+}$. On ionisation, transition-block atoms always lose their $4s$ electrons before any $3d$ electrons, because once the $3d$ subshell contains electrons it drops to a lower energy than $4s$, making the $4s$ electrons the ones most easily removed: $\text{Zn}^{2+}:\ [\text{Ar}]3d^{10}$

So both the atom and its only stable ion have a completely full $3d^{10}$ subshell, never a partly-filled one.

Step 2: Apply the definition of a transition element

A transition element is defined as a d-block element that forms at least one stable ion with a partly-filled (incomplete) d subshell, neither completely empty ($d^0$) nor completely full ($d^{10}$). Because zinc’s only ion, $\text{Zn}^{2+}$, has a full $3d^{10}$ subshell, zinc fails this test and is not a transition element, even though it sits in the d-block.

(This is exactly analogous to scandium, whose only ion, $\text{Sc}^{3+}$, has an empty $3d^0$ subshell. Also excluded, but for the opposite reason: too few d electrons rather than too many.)

Step 3: Evaluate each option

• A: correctly gives $\text{Zn}: [\text{Ar}]3d^{10}4s^2$ and $\text{Zn}^{2+}: [\text{Ar}]3d^{10}$, and correctly links the full d subshell to the absence of transition-element behaviour. This matches the reasoning above.
• B: incorrect. It is the $4s$ electrons, not $3d$ electrons, that are lost first on ionisation; the stated configuration $[\text{Ar}]3d^{8}4s^2$ for $\text{Zn}^{2+}$ is wrong on two counts (wrong electrons removed, and wrong total electron count for a $2+$ ion).
• C: incorrect. The $3d$ subshell in zinc is completely full (10 electrons), not empty; this option describes the scandium/$d^0$ case, not the zinc/$d^{10}$ case.
• D: incorrect reasoning. The formal definition of a transition element concerns the d-subshell occupancy of its ions, not whether variable oxidation states are shown (though the two facts are related in practice, having a full d subshell is the actual reason zinc is excluded, not the number of oxidation states it displays).

Final answer

• Zinc is d-block but not a transition element because its only ion, $\text{Zn}^{2+}$, has a completely full $3d^{10}$ subshell rather than a partly-filled one, option A.