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

\(\mathrm{Ni}^{2+}\) exhibits color in aqueous solution.

Step 1: Understanding the Problem

We need to determine which of the given ions exhibits color in aqueous solution. The color in transition metal ions is often due to electron transitions between d-orbitals.

Step 2: Review Electron Configuration

Consider the electron configuration of each ion. Transition metals typically show color due to d-d transitions, which require at least one unpaired electron in d-orbitals.

Step 3: Analyzing Each Ion

(a) \(\mathrm{Ti}^{4+}\) has no electrons in its d-orbitals. (b) \(\mathrm{Zn}^{2+}\) has a full \(d^{10}\) configuration, so no d-d transitions. (c) \(\mathrm{Ni}^{2+}\) has a \(d^8\) configuration, allowing for d-d transitions. (d) \(\mathrm{Sc}^{3+}\) has no electrons in its d-orbitals.

Step 4: Identify the Ion with Unpaired Electrons

\(\mathrm{Ni}^{2+}\) has two unpaired electrons in its \(d\)-orbitals, which can absorb visible light for d-d transitions, resulting in color.

Key Concepts

Color in Aqueous Solutions

Transition metals often exhibit vibrant colors when dissolved in aqueous solutions. This coloration is primarily due to the interaction between the metal ions and the surrounding water molecules. When these metal ions are in solution, they typically form complex ions where water molecules coordinate to the metal center. The specific colors we observe arise from the electron transitions within the d-orbitals of the metal ions.

These transitions are influenced by the energy levels of the d-orbitals, which split in the presence of ligands like water. As a result, certain wavelengths of visible light are absorbed. The color that we see is the complementary color of the absorbed light, giving each transition metal a unique hue in solution.

Electron Configuration

The electron configuration of an atom or ion is a crucial concept when analyzing their chemical properties, including color in transition metals. It refers to the distribution of electrons in an atom's orbitals and can be used to predict the potential for electron transitions.

For transition metals, the focus is on the d-orbitals, as they play a significant role in the ion's color. When a transition metal is ionized, it often loses its s-electrons first, followed by its d-electrons.

• Understanding the electron configuration aids in predicting whether an ion will have unpaired electrons in its d-orbitals, which is crucial for color.
• For example, ions with completely filled or empty d-orbitals typically do not show color since there are no possible d-d electronic transitions.

In summary, examining the electron configuration allows us to determine the presence of unpaired electrons and predict if an ion will display color in solution.

d-d Transitions

d-d transitions are the electronic transitions between d-orbitals that give rise to color in transition metal compounds. These occur when electrons jump from one d-orbital to another within the same energy level but split into different energy states due to the crystal field effect.

When light shines on the solution containing transition metal ions, certain wavelengths are absorbed as electrons are promoted from lower to higher energy d-orbitals. The energy difference between these orbitals determines which wavelength is absorbed, directly affecting the color observed.

• d-d transitions require the d-orbitals to be partially filled; a fully filled or empty set of d-orbitals will not exhibit such transitions.
• The presence of unpaired electrons is necessary for these transitions to occur.

Thus, understanding d-d transitions is fundamental in explaining why certain transition metals display color in their aqueous solutions.

Unpaired Electrons

Unpaired electrons in d-orbitals are a key factor in the color exhibited by transition metal ions. When an electron is unpaired, it means there is a single electron not paired with another opposite-spin electron in an orbital.

These unpaired electrons are crucial for d-d transitions to take place, as they provide the possibility for electrons to move between different energy d-orbitals. The more unpaired electrons an ion has, the more potential for varied transitions, often leading to a richer intensity of color in the solution.

• The presence of unpaired electrons in ions like \(\mathrm{Ni}^{2+}\), which has a \(d^8\) configuration, allows for multiple d-d transitions, visible as distinct colors.
• Ions lacking unpaired electrons, such as \(\mathrm{Zn}^{2+}\) with a \(d^{10}\) configuration, will not exhibit color through d-d transitions.

Thus, the presence of unpaired electrons is essential for the manifestation of color due to d-d transitions in transition metal ions.