Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Cuprous ion is colourless, while cupric ion is coloured, because (a) cuprous ion has incomplete d orbital and cupric ion has a complete d orbital (b) cuprous ion has a complete d orbital and cupric ion has incomplete d orbital (c) cuprous ion has half filled d orbital (d) cupric ion has unpaired electrons in d orbital

Step-by-Step Solution

Answer

Cupric ion is coloured because it has unpaired electrons in d orbitals, while cuprous ion is colourless because its d orbitals are complete.

Step 1: Understand the Ion Configurations

Cuprous ion, Cu⁺, is formed when copper loses one electron, resulting in a configuration of ext{(1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰)}. Cupric ion, Cu²⁺, is formed when copper loses two electrons, resulting in a configuration of ext{(1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁹)}.

Step 2: Identify the Key Characteristic of Colour

Cupric ion ( ext{Cu}^{2+}) has one less electron in the d orbital, which results in an incomplete d orbital ( ext{3d⁹}). The presence of unpaired electrons in its d orbital leads to the absorption of certain wavelengths of light, making it appear coloured.

Step 3: Determine the Colour of Cuprous Ion

Cuprous ion ( ext{Cu}^+), having a complete d subshell ( ext{3d¹⁰}), has all paired electrons and does not absorb wavelengths in the visible spectrum significantly, making it appear colourless.

Step 4: Choose the Correct Statement

The statement that correctly describes why cupric ion is coloured and cuprous ion is not is: "cupric ion has unpaired electrons in d orbital" (option d).

Key Concepts

Cuprous Ion

The cuprous ion, often represented as Cu⁺, originates from a copper atom losing one electron. This ionic form maintains a specific electron configuration:

• This configuration is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10}\).

Copper's unique behavior in forming different ions stems from the transition metal's ability to lose electrons from its 4s and sometimes from its 3d orbitals. When it forms the Cu⁺ ion, the 3d subshell is completely filled.
This complete filling means all d-orbital electrons are paired, leading to its lack of color. Therefore, the cuprous ion does not significantly absorb light within the visible spectrum, rendering it colorless in appearance.

Cupric Ion

The cupric ion, symbolized as Cu²⁺, comes into existence when copper atoms lose two electrons. This results in a different electron arrangement:

• The electron configuration for Cu²⁺ is \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^9\).

With only nine electrons in the 3d orbitals, the cupric ion showcases one electron short of a stable filled configuration, leaving an unpaired electron.
This unpaired electron is crucial because it leads to the cupric ion's ability to absorb specific wavelengths of visible light. This absorption causes the excited state in the electrons, which results in the coloration of the solution containing these ions. The colors vary depending on additional ligands and the environment around the ion.

Electron Configuration

Electron configuration is fundamental to understanding chemical properties in chemistry. It details how electrons are distributed in an atom's orbitals. Specifically, for transition metals like copper, electron configuration reveals significant insights:

• The loss or gain of electrons in these metals often involves their s and d orbitals, resulting in unique chemical behavior and formation of colored complexes.
• For example, transition metals like copper exhibit a variety of electron configurations when they become ions (e.g., Cu⁺ with \(3d^{10}\) and Cu²⁺ with \(3d^9\)).

Understanding these configurations helps predict ion behavior and the resultant physical properties, such as color, magnetic properties, and bonding behavior.

D Orbital

The d orbitals in transition metals like copper are partly responsible for their unique properties, including color and reactivity. Transition metals have five d orbitals ( d_{xy}, d_{yz}, d_{zx}, d_{x^2-y^2}, and d_{z^2} ) that can hold a total of 10 electrons:

• Each orbital can hold up to two electrons, which can be paired or unpaired, impacting the metal's chemistry and physical characteristics.
• Unpaired electrons in these orbitals contribute greatly to the metal's visible color and magnetic properties.

In the case of copper ions:
Cu⁺ has all ten electrons paired, while Cu²⁺ has nine electrons, leaving one unpaired. This unpaired nature in Cu²⁺ allows for light absorption that results in the typical colored appearance of many cupric compounds.