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

Cr^{3+} imparts color to an aqueous solution.

Step 1: Identify the ion with unpaired d-electrons

Colored aqueous solutions generally result from the presence of transition metal ions with unpaired electrons in their d-orbitals. To determine which ion imparts color, we check for unpaired d-electrons in each option.

Step 2: Analyze ion Cr^{3+}

Cr^{3+} has an atomic number of 24 and typically loses three electrons, resulting in a 3d^3 electron configuration. This has three unpaired d-electrons, suggesting it can impart color.

Step 3: Analyze ion Cu^+

Cu has an atomic number of 29, and Cu^+ loses one electron, resulting in an electronic configuration of 3d^{10}. Since all d-electrons are paired, Cu^+ does not impart color.

Step 4: Analyze ion Ti^{4+}

Ti^{4+}, with an atomic number of 22, loses four electrons. This results in a 3d^0 electron configuration. With no d-electrons, it cannot impart color.

Step 5: Analyze ion Zn^{2+}

Zn has an atomic number of 30, and Zn^{2+} will lose two electrons, resulting in a 3d^{10} configuration. All d-electrons are paired, so Zn^{2+} does not impart color.

Step 6: Conclusion: Determine the color-imparting ion

Among the given options, only Cr^{3+} has unpaired d-electrons, which can interact with light to impart color to a solution.

Key Concepts

Unpaired d-electrons

Transition metal ions can often impart colors to solutions due to the presence of unpaired d-electrons. These electrons are found in the d-orbital of the transition metals. When light interacts with these ions in solution, the energy from the light can cause electron transitions. As electrons get excited, they move between different energy levels within the d-orbitals. This energy absorption and subsequent release when they return to their original state is what causes the solution to appear colored. The color we perceive corresponds to the specific wavelengths of light that are absorbed and those that are reflected. Therefore, ions like

• ext{Cr}^{3+} with unpaired d-electrons will likely impart color to a solution because there are available unpaired electrons ready to interact with incoming light.

Electronic configuration

Understanding electronic configurations is crucial when predicting whether an ion will impart color. Electronic configurations detail the distribution of electrons in an atom's or ion's orbitals. Transition metals like chromium, copper, titanium, and zinc have unique configurations that determine their chemical behavior. When these metals form ions, they usually lose electrons from their outermost shells. For example:

• Cr atom: ewline ext{[Ar] 3d}^5 ewline ext{4s}^1
• ext{Cr}^{3+} ion: 3d^3

Here, the ext{3d}^3 configuration shows there are unpaired electrons, making it able to impart color. By checking for unpaired d-electrons, we can predict which ions are likely to produce colored solutions. For example,

• The ext{Cu}^+ ion has a ext{3d}^{10} configuration where all d-electrons are paired, making it unlikely to produce color.

Color of solutions

The vibrant colors seen in aqueous solutions of transition metal ions are fascinating phenomena. These colors are directly influenced by the d-electron configurations within the metal ions. When light passes through the solution, certain wavelengths are absorbed while others are transmitted or reflected, giving rise to the observed color. For example, a solution containing ext{Cr}^{3+} ions might appear greenish due to such electron transitions and light interactions. Whether or not the transition metal ion imparts a color is greatly determined by:

• The presence of unpaired d-electrons: An essential requirement for light absorption and color display.
• The specific electronic configuration: Dictates the particular wavelengths absorbed and the resultant color perceived.

Thus, transition metal ions with complete d-orbitals, where electrons are all paired, generally result in colorless solutions because there are few electron transitions. This concept helps explain why some substances are colorful while others remain colorless.