Question

In which of the following pairs are both the ions coloured in aqueous solution? (a) \(\mathrm{Sc}^{3+}, \mathrm{Co}^{2+}\) (b) \(\mathrm{Ni}^{2+}, \mathrm{Cu}^{+}\) (c) \(\mathrm{Ni}^{2+}, \mathrm{Ti}^{3+}\) (d) \(\mathrm{Sc}^{3+}, \mathrm{Ti}^{3+}\) (Atomic number \(\mathrm{Sc}=21, \mathrm{Ti}=22, \mathrm{Ni}=28, \mathrm{Cu}=29\) \(\mathrm{Co}=27\) )

Short answer

Option (c): Ni^{2+}, Ti^{3+} are both colored in aqueous solution.

Step-by-step solution

Understanding Color in Ions

Colored ions generally have unpaired electrons in d-orbitals. In an aqueous solution, these electrons can absorb certain wavelengths of light, causing color to be emitted. We need to determine which ions have unpaired d-electrons.

Analyzing Option (a): Sc^{3+}, Co^{2+}

Sc^{3+} has the electron configuration [Ar], which means it doesn't have any d-electrons, and cannot be colored. Co^{2+} has the configuration [Ar] 3d^7, which means it has unpaired d-electrons and can be colored.

Analyzing Option (b): Ni^{2+}, Cu^{+}

Ni^{2+} has the electron configuration [Ar] 3d^8 and can be colored due to unpaired electrons. Cu^{+}, however, has a complete d^10 configuration in an aqueous solution, so it does not show color.

Analyzing Option (c): Ni^{2+}, Ti^{3+}

Ni^{2+} has the electron configuration [Ar] 3d^8, indicating it's colored due to unpaired electrons. Ti^{3+} has the configuration [Ar] 3d^1, which also has unpaired electrons, so it is colored.

Analyzing Option (d): Sc^{3+}, Ti^{3+}

Sc^{3+}, as previously mentioned, has no d-electrons and thus is not colored. Ti^{3+} on the other hand has a 3d^1 configuration and shows color.

Selecting the Correct Option

From all the analyses, the pair in option (c)  Ni^{2+}, Ti^{3+}  have both ions that display color due to the presence of unpaired d-electrons in their respective configurations.

Key concepts

Color in Ions

Transition metal ions are known for their vibrant colors in aqueous solutions. This fascinating phenomenon is primarily due to their electronic structure, especially the arrangement of electrons in the d-orbitals. When these ions dissolve in water, the d-electrons can absorb specific wavelengths of light, causing them to jump to higher energy levels.
When these electrons return to their original energy levels, they emit light of certain colors.
For an ion to show color, it generally needs to have unpaired electrons in its d-orbitals. Hence, the color you'll observe is the complementary color of the light absorbed.

• Unpaired Electrons: Ions like \( \text{Ni}^{2+} \) which have unpaired electrons, are capable of exhibiting color.
• Paired Electrons: Ions such as \( \text{Cu}^{+} \) which have all their d-electrons paired, do not absorb visible light in a way that produces color. Instead, they appear colorless in water.

Recognizing whether an ion will be colored in solution involves understanding its electron configuration. Only those with unpaired d-electrons will display color.

Unpaired Electrons

The presence of unpaired electrons in the d-orbitals is a critical factor that determines the color of transition metal ions. Unpaired electrons in these orbitals allow the ions to absorb visible light.Let's delve deeper into the behavior of these electrons:

• Configuration and Colors: In the case of \( \text{Co}^{2+} \), its electron configuration is \([\text{Ar}] 3d^7\), leaving unpaired electrons that attribute to its colorful nature.
• Energy Transitions: As non-paired d-electrons absorb energy, they transition between energy levels, leading to light absorption and the emission of colored light.

The diversity of possible electronic transitions, and variety of subsequent colors observed, aligns with the number of unpaired electrons and their arrangement within the d-orbitals.

Aqueous Solutions

When transition metal ions dissolve in water, they interact with the solvent in a unique way, often resulting in vibrant colors due to their unpaired d-electrons.In an aqueous solution, the following processes occur:

• Solvation: Water molecules surround the ion, forming a complex in which the energy of the d-orbitals is split into different energy levels. This is known as the 'ligand field' effect.
• Light Absorption: The split in energy levels allows specific wavelengths of visible light to be absorbed as the electrons jump to higher levels.

Hence, ions like \( \text{Ni}^{2+} \) and \(\text{Ti}^{3+}\) , with unpaired d-electrons, result in observable colors in a clear water solution. This characteristic is widely used in chemical identification and analysis, illustrating the practical importance of this chemical behavior.