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

(c)
}{2+} is colored due to unpaired electrons.

Step 1: Identify Electron Configuration

To determine whether an ion exhibits color in aqueous solution, check its electron configuration. Transition metal ions with unpaired electrons are typically colored due to d-d transitions. Let's find the electron configurations for the ions. -
0^{4+}": Electron configuration ends in [Ar] (no d-electrons). -
}{2+}": Electron configuration ends in [Ar]3d^{10} (no unpaired electrons). -
}{2+}": Electron configuration ends in [Ar]3d^{8} (unpaired electrons present). -
}{3+}": Electron configuration ends in [Ar] (no d-electrons).

Step 2: Determine Color Presence

Transition metal ions exhibit color if they have unpaired d-electrons. From the electron configurations: -
^4+
}{2+}
}{3+}
}{2+}
}{8} configuration, indicating unpaired electrons, which are needed for color.

Step 3: Select the Ion with Color

Based on the analysis,
}{2+} has unpaired electrons, making it the only ion that exhibits color. Therefore,
}{2+} is colored in aqueous solution.

Key Concepts

Electron Configuration

Understanding electron configuration is key to knowing why certain ions display color in solution. Electrons are arranged in an atom in energy levels or shells, with each shell further divided into subshells and orbitals. Notably, the d-block elements, known as transition metals, can have partially filled d orbitals. When these d orbitals contain electrons, they can interact with light, leading to color. For ions:

• Check their electron configurations by writing them out from their atomic numbers.
• Remove electrons according to their charge (typically starting from the outermost shell first), which usually means losing s orbital electrons before d electrons.

In the given exercise,

• ext{ ext{Ti}^{4+}}: No d electrons (similar electronic configuration to Argon),
• ext{ ext{Zn}^{2+}}: Full d orbital (3d^{10}),
• ext{ ext{Ni}^{2+}}: Partially filled d orbitals (3d^{8}),
• ext{ ext{Sc}^{3+}}: No d electrons (similar electronic configuration to Argon),

which influences whether they can exhibit color. Only ions with unpaired electrons, like ext{ ext{Ni}^{2+}}, are generally colorful.

Transition Metal Ions

Transition metal ions are fascinating due to their unique electronic structures. They are elements found in the d-block of the periodic table, specifically from groups 3 to 12. These metals often form colored compounds, especially in ion form, due to their partially filled d orbitals. The presence of unpaired d electrons is crucial for their colorful properties. When we consider ions such as

• ext{Fe}^{2+}
• ext{Cu}^{2+}
• ext{Co}^{2+}

all have partially filled d orbitals with unpaired electrons, which means they can form a wide variety of colors in solution. For transition metal ions, certain key points are:

• The electrons can absorb visible light energy, transitioning between different d-orbital energy levels.
• This absorbed energy can change which light is reflected, making solutions appear colored.

Therefore, the presence of unpaired electrons in transition metal ions like ext{ ext{Ni}^{2+}} causes color, contrasting with ions that lack these unpaired electrons and therefore appear colorless.

d-d Transitions

d-d transitions are a fascinating phenomenon explaining the colors seen in transition metal ions. A d-d transition occurs when an electron jumps from a lower energy d orbital to a higher energy d orbital. Here's how it works:

• The electrons in the d orbitals of transition metals absorb specific wavelengths of light to transition to a higher energy level.
• When specific wavelengths are absorbed, the complementary colors are reflected, and that's the color we see.
• This transition is only possible with the presence of unpaired d electrons, a trait specific to many transition metals.

In simpler terms, light absorption involves electrons jumping within the d orbitals:

• When light hits a transition metal ion in a solution, electronic transitions between d orbitals can occur if conditions are right (unpaired electrons present).
• The absorbed part of the spectrum determines the color appearing to our eyes.

The ability of ext{ ext{Ni}^{2+}} to exhibit color is due to such unpaired d electrons facilitating these d-d transitions.