Which aqueous solution of an ion is likely to be coloured based on the electron configuration of that ion?
Correct Answer: \( \boxed{\mathrm{A}} \)
Transition metal ions are often coloured when they contain partially filled \(d\)-orbitals, allowing \(d\)-\(d\) electronic transitions to occur.
For \( \mathrm{Ni^{2+}} \), the electron configuration is:
\( \mathrm{Ni}: [Ar]\,3d^8\,4s^2 \)
Therefore:
\( \mathrm{Ni^{2+}}: [Ar]\,3d^8 \)
The \(3d\) subshell is partially filled, so \(d\)-\(d\) transitions can occur and the aqueous solution is likely to be coloured.
In contrast, \( \mathrm{Sc^{3+}} \), \( \mathrm{Cu^+} \), and \( \mathrm{Zn^{2+}} \) have \(d^0\) or \(d^{10}\) configurations, so \(d\)-\(d\) transitions are not possible.
Therefore, the correct answer is (A).
Which statements best explain why copper(II) ions, \( \mathrm{Cu^{2+}(aq)} \), are blue in aqueous solution?
I. Copper(II) ions absorb orange light.
II. Blue light is emitted as electrons move to lower orbitals.
III. Electrons are promoted between \(d\)-orbitals.
Correct Answer: \( \boxed{\mathrm{B}} \)
Copper(II) ions have partially filled \(d\)-orbitals. In an aqueous complex, the \(d\)-orbitals split into different energy levels.
Electrons can absorb visible light and become promoted from a lower-energy \(d\)-orbital to a higher-energy \(d\)-orbital:
\( \mathrm{d_{lower}+h\nu\rightarrow d_{higher}} \)
The orange region of visible light is absorbed, so the complementary colour, blue, is observed.
Statement II is incorrect because the blue colour is not produced by emission of blue light as electrons fall back to lower orbitals. The observed colour results primarily from selective absorption of visible light.
Therefore, statements I and III are correct, so the correct answer is (B).
In the fourth energy level of an atom, the sub-levels increase in energy in the following order: \({s} < {p} < {d} < {f}\).
This is based on the general Aufbau principle, where the complexity of orbitals and the distance of electrons from the nucleus increase from s → p → d → f.
Therefore, the correct answer is:
✅ Answer: (D)
