Problem 23 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following ions exhibits colour in aqueous solution? (a) \(\mathrm{Ti}^{4+}\) (b) \(\mathrm{Zn}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Sc}^{3+}\)

Step-by-Step Solution

Answer

The ion cgreenNi^cfifty2+cfifty exhibits color in aqueous solution.

Step 1: Understanding the Formation of Color in Ions

Ions in aqueous solution are colored due to the presence of unpaired electrons in their d-orbitals. The transition of these electrons results in the absorption of certain wavelengths of light, which gives rise to color.

Step 2: Analyzing Each Ion

Examine the electronic configuration of each ion: (a)
4_{ ext{Ti}} = 22,
4_{ ext{Ti}^{4+}} = 18, configuration: 1s²2s²2p⁶3s²3p⁶ (no d electrons) (b)
4_{ ext{Zn}} = 30,
4_{ ext{Zn}^{2+}} = 28, configuration: 1s²2s²2p⁶3s²3p⁶3d¹⁰ (filled d-shell) (c)
4_{ ext{Ni}} = 28,
4_{ ext{Ni}^{2+}} = 26, configuration: 1s²2s²2p⁶3s²3p⁶3d⁸ (partially filled d-shell) (d)
4_{ ext{Sc}} = 21,
4_{ ext{Sc}^{3+}} = 18, configuration: 1s²2s²2p⁶3s²3p⁶ (no d electrons)

Step 3: Determining Coloration

Only ions with partially filled d-orbitals can exhibit color due to d-d transitions. From our analysis, only fiftyNi^cfifty(API)^fifty2+cfifty has partially filled d-orbitals.

Key Concepts

Coloration in Ions

Many transition metal ions display vibrant colors when dissolved in water. This beautiful phenomenon is primarily due to the peculiar nature of these ions' d-orbitals.
When a transition metal ion is placed in an aqueous solution, the interaction with water molecules modifies the energy levels of the d-orbitals. It creates an energy gap between sub-orbitals.
This situation allows the ion to absorb specific wavelengths of visible light. The remaining wavelengths, which are not absorbed, are transmitted or reflected, providing the solution with its characteristic color.
For instance, when an ion has available d-orbital spaces, electrons can jump from a lower d-orbital to a higher one, a process known as "d-d transition."

• Only metals with partially filled d-orbitals can exhibit these colorful transitions.
• Different ions will absorb different wavelengths depending on their specific electronic configurations and the presence of ligands in the solution.

Hence, ions like Ni^{2+} among the options listed, possess unpaired d-electrons that can undergo these transitions, leading to visible coloration.

d-orbital Transitions

In transition metal ions, d-orbitals can participate in transitions that result in color changes. But, what exactly are these transitions?
When light strikes an ion, some electron transitions may occur due to absorption of energy. This happens specifically if the ion has unpaired electrons in its d-orbitals.
These d-orbitals, split into higher and lower energy levels due to the presence of ligands like water molecules, induce what's called "crystal field splitting."
Electrons can "jump" from a lower energy d-orbital to a higher one. The energy associated with this jump corresponds to certain light wavelengths, primarily in the visible spectrum.

• Electrons absorbing light energy can lead to colorful solutions.
• The size of the jump (or energy gap) is influenced by surroundings and types of ligands.

The color we perceive is essentially the complementary color of the wavelength absorbed by the electron transition. In short, if a color depends on a partially filled d-orbital, it enables fascinating coloration through d-d transitions.

Electronic Configuration of Ions

The electronic configuration of an ion is highly influential in determining whether it will exhibit color. Analyzing this configuration helps predict which ions are colorful in aqueous solutions.
Transition metals like nickel often exhibit colorful interactions in solutions due to their specific electronic configurations.
For instance, the Ni^{2+} ion (with a configuration 1s²2s²2p⁶3s²3p⁶3d⁸) has a partially filled 3d orbital, allowing electrons to move between d-orbitals and result in coloration.
In contrast, ions such as Ti^{4+} or Sc^{3+} have empty or completely filled d-orbitals, leaving no room for d-d transitions.

• For coloration, ions must have unpaired electrons in their d-orbitals.
• The electronic configuration is essential for predicting an ion's properties.

Simply put, understanding the electronic configuration of ions is key to understanding why only some ions, like nickel ions, are colorful in aqueous solutions.