Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following ions is colourless in its aqueous solution? (a) \(\mathrm{Ti}^{3+}\) (b) \(\mathrm{Cu}^{2+}\) (c) \(\mathrm{Ni}^{2+}\) (d) \(\mathrm{Zn}^{2+}\)

Step-by-Step Solution

Answer

The colorless ion is (d) \(\mathrm{Zn}^{2+}\).

Step 1: Understanding color in transition metal ions

Transition metal ions often have colors due to d-d electron transitions. An ion will be colorless if there are no such transitions possible, which often occurs when the d-orbitals are either completely filled or empty.

Step 2: Analyzing electron configurations

We need to determine the electron configurations of the given ions. \(\mathrm{Ti^{3+}}\) has the configuration \([\mathrm{Ar}]\ 3d^1\), \(\mathrm{Cu^{2+}}\) has \([\mathrm{Ar}]\ 3d^9\), \(\mathrm{Ni^{2+}}\) has \([\mathrm{Ar}]\ 3d^8\), and \(\mathrm{Zn^{2+}}\) has \([\mathrm{Ar}]\ 3d^{10}\).

Step 3: Identifying the colorless ion

Colorlessness in transition metal ions occurs when there are no d-d transitions. \(\mathrm{Zn^{2+}}\) has a \(3d^{10}\) configuration, meaning it has a completely filled d-subshell, preventing d-d transitions.

Key Concepts

d-d Electron Transitions

Transition metal ions often exhibit fascinating colors due to what are known as d-d electron transitions. These transitions occur when electrons in the d-orbitals of a metal ion absorb specific wavelengths of light, then jump from a lower energy level to a higher energy level. This absorption of light at particular wavelengths results in the complementary color being observed. For example, if an ion absorbs light in the red part of the spectrum, it might appear green.

However, not all transition metal ions will show color due to these transitions. When there are no available lower energy d-orbitals for the d-electrons to jump to, due to either completely empty or completely filled d-orbitals, these transitions cannot occur, and the ion does not absorb visible light—it appears colorless.

Electron Configurations

The electron configuration of an ion tells us how electrons are distributed among the orbitals. For transition metals, the interesting part often lies in the d-orbitals. Knowing the electron configuration helps determine if d-d transitions are possible, which in turn helps predict if the ion will be colored or colorless.

Here’s a brief look at the configurations of the ions from the exercise:

• ** Ti^{3+} **: The configuration is [ Ar ] 3d ^1^**. Since there is an unpaired electron, the potential for d-d transitions exists.
• **Cu^{2+}**: With the configuration [Ar] 3d^9, there are also unpaired electrons present, allowing for possible d-d transitions.
• **Ni^{2+}**: This ion has [Ar] 3d^8, which similarly allows for unpaired electrons capable of transitions.
• **Zn^{2+}**: Finally, with a configuration of [Ar] 3d^10, the d-orbitals are completely filled, leaving no room for electron transitions.

Colorlessness in Ions

The color of a transition metal ion in solution is greatly affected by its electron configuration and the possibility of d-d transitions. An ion becomes colorless when it either has no d-electrons or has a completely filled d-subshell.

For instance, Zn^{2+} in our list is colorless because its 3d orbitals are completely filled at 3d^{10}. This full occupancy means there are no partially filled orbitals for electrons to transition between, disabling the ability to absorb visible light.

In contrast, ions like Ti^{3+}, Cu^{2+}, and Ni^{2+} have partially filled d-orbitals, hence exhibiting color due to their potential for d-d transitions. Understanding these principles explains why not all metal ions are colorful and when you'll encounter a clear solution.