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 in aqueous solution is \( \mathrm{Zn}^{2+} \).

Step 1: Identify the Key Information

We are asked to find out which ion is colorless in its aqueous solution from the given options: \( \mathrm{Ti}^{3+} \), \( \mathrm{Cu}^{2+} \), \( \mathrm{Ni}^{2+} \), and \( \mathrm{Zn}^{2+} \). Colorless ions typically have no unpaired electrons in their d-orbitals.

Step 2: Analyze Electronic Configurations

Let's analyze the electron configuration of each ion: - \( \mathrm{Ti}^{3+} \) ion lacks 1 electron from its d-orbitals and usually has colored compounds due to unpaired electrons.- \( \mathrm{Cu}^{2+} \) generally has unpaired d-electrons and exhibits colored complexes.- \( \mathrm{Ni}^{2+} \) typically contains unpaired d-electrons, resulting in colored compounds.- \( \mathrm{Zn}^{2+} \) has a 3d\(^{10} \) electron configuration with all paired electrons, making it colorless.

Step 3: Conclude Based on Electronic Configuration

From the analysis of electronic configurations, we can conclude that \( \mathrm{Zn}^{2+} \) is the only ion with completely filled d-orbitals, resulting in its colorless nature in aqueous solution.

Key Concepts

Electronic Configuration

Electronic configuration is the distribution of electrons of an atom or molecule in atomic or molecular orbitals. It is fundamental in predicting the chemical, physical, and electrical properties of elements. Generally, for a given element, electrons fill orbitals in a specific order, guided by the Aufbau principle, Hund's rule, and the Pauli exclusion principle.

• Aufbau Principle: This principle states that electrons occupy the lowest energy orbitals available. Electrons fill orbitals starting from the lowest energy level to the highest.
• Hund's Rule: Electrons will fill an unoccupied orbital before they pair up. This minimizes the electron-electron repulsions within an atom.
• Pauli Exclusion Principle: No two electrons can have the same set of four quantum numbers in an atom. Essentially, an orbital can hold a maximum of two electrons with opposite spins.

Knowing the electronic configuration of ions helps predict their reactivity and color. For example, a full or empty d-subshell often leads to the formation of colorless compounds, as no d-d transitions are possible.

Transition Metals

Transition metals are elements found in the center of the periodic table, specifically in the d-block, which includes groups 3 to 12. These metals are known for forming colored compounds, primarily due to their partially filled d-orbitals. Transition metals are characterized by the following:

• Variable Oxidation States: Transition metals can form ions with different charges by losing different numbers of d-electrons. This property allows them to form a variety of compounds.
• Colored Compounds: This arises from d-d electron transitions within the d-orbitals when light is absorbed.
• Complex Formation: Transition metals tend to form complex ions with a set number of molecules or ions surrounding them, leading to versatile chemical behavior.

Unlike main group metals, transition metals display significant diversity in their chemical behavior due to the variable oxidation states and complex formation capacity. With few exceptions, transition metal ions show colors in solutions due to these unique electronic transitions.

Unpaired Electrons

Unpaired electrons refer to electrons that occupy an orbital alone, without a paired electron of opposite spin. In the context of transition metals, the presence of unpaired electrons in d-orbitals is a key reason for the formation of colored ions.

• Magnetism: Unpaired electrons contribute to the magnetic properties of transition metal compounds. The presence of unpaired electrons can make a substance paramagnetic, meaning it is attracted by a magnetic field.
• Color of the Compounds: When transition metals have unpaired d-electrons, exposure to light can cause these electrons to jump between d-orbitals, absorbing particular wavelengths of light and imparting color to the compounds.
• Identifying Colorless Ions: Ions such as \( \mathrm{Zn}^{2+} \), with a 3d\(^{10} \) configuration, are colorless because there are no available d-d transitions, owing to all electrons being paired.

Understanding the behavior of unpaired electrons provides insight into why certain ions are colored, while others, with all paired electrons like \( \mathrm{Zn}^{2+} \), remain colorless in solutions.