Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following ionic species will impart colour to an aqueous solution? (a) \(\mathrm{Cr}^{3+}\) (b) \(\mathrm{Cu}^{+}\) (c) \(\mathrm{Ti}^{4+}\) (d) \(\mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

\( \mathrm{Cr}^{3+} \) will impart color to the solution.

Step 1: Understand Color Formation in Ionic Species

Ions impart color to a solution if they have unpaired d-electrons. Transition metal ions are typically colorful due to d-d electron transitions.

Step 2: Analyze Each Option's Electron Configuration

- \( \mathrm{Cr}^{3+} \) has a 3d\(^{3}\) configuration with unpaired electrons.- \( \mathrm{Cu}^{+} \) has a 3d\(^{10}\) configuration without unpaired electrons.- \( \mathrm{Ti}^{4+} \) has no 3d electrons as it loses all its d-electrons.- \( \mathrm{Zn}^{2+} \) has a 3d\(^{10}\) configuration without unpaired electrons.

Step 3: Determine the Ion with Unpaired Electrons

Identify that \( \mathrm{Cr}^{3+} \) is the only ion with unpaired d-electrons (3d\(^{3}\)) which can undergo electron transitions and impart color to the solution.

Key Concepts

Color of Ionic Species

The vibrant color of many ionic species, especially those involving transition metals, is a fascinating phenomenon associated with their electronic structure. Ions that impart color to solutions are often found in compounds involving transition metals. This color arises due to specific interactions at the atomic level.

For an ion to display color, it typically must possess unpaired electrons in its d-orbitals. These unpaired electrons allow for transitions from one d-orbital to another. When light interacts with these ions, it can cause electron transitions within the d-orbital, which absorb specific wavelengths of light.

The remaining light that is transmitted or reflected gives the ion its characteristic color. The presence or absence of unpaired d-electrons is, therefore, crucial. Without them, such transitions (and colors) are usually not possible.

Electron Configuration

Electron configuration is a fundamental concept that helps us understand where electrons are located around an atom. It is essential in determining the chemical properties of an element, including coloration in transition metal ions.

Transition metal ions are unique because they have partially filled d-orbitals. These d-orbitals can hold a maximum of 10 electrons, allowing for several unique configurations:

• For example, a \(\mathrm{Cr}^{3+}\) ion has a 3d\(^{3}\) electron configuration with three unpaired electrons, which makes it potentially colorful.
• On the other hand, ions like \(\mathrm{Cu}^{+}\) and \(\mathrm{Zn}^{2+}\) feature a 3d\(^{10}\) configuration, lacking unpaired electrons, and thus typically do not show color.

Negotiating the removal and arrangement of these electrons in transition metals is key to understanding the role they play in coloration. The presence of unpaired electrons in the d-orbitals is a significant factor that contributes to the potential for electron transitions, affecting the visual appearance of these ions.

d-d Electron Transitions

One of the main reasons why transition metal ions are colorful is due to d-d electron transitions. But what exactly does this mean? When an electron in a d-orbital absorbs energy, it can move to a higher energy d-orbital. This jump between levels is what is known as a d-d transition.

The energy required for these transitions falls in the visible spectrum of light. When electrons absorb specific wavelengths of light to transition to another d-level, the ion reflects the complementary color of the absorbed light.

For instance, in \(\mathrm{Cr}^{3+}\), the presence of unpaired d-electrons allows these transitions to occur, giving rise to its distinct color. Meanwhile, if all d-orbitals are fully occupied or empty, like in \(\mathrm{Cu}^{+}\) or \(\mathrm{Zn}^{2+}\) respectively, no such transitions are possible, and the ions remain colorless.

Hence, identifying the d-electron configuration tells us a lot about the possible colors an ion might exhibit and highlights the amazing interplay between light and matter in transition metals.