Problem 2 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of these ions is coloured as well as paramagnetic in nature? (a) \(\mathrm{Cu}^{2+}\) (b) \(\mathrm{Cu}^{+}\) (c) \(\mathrm{Nl}^{2+}\) (d) Both a and c

Step-by-Step Solution

Answer

(d) Both a and c (7Cu^{2+} and 7Ni^{2+})

Step 1: Understanding Paramagnetism

Paramagnetism occurs when an ion has unpaired electrons in its d-orbitals. These unpaired electrons have magnetic moments that add up to create a net magnetic moment, making the ion paramagnetic.

Step 2: Understanding Ion Coloration

Ions absorb specific wavelengths of light due to electronic transitions between d-orbitals, which creates colors. The ions with partially filled d-orbitals can show colors due to these d-d transitions.

Step 3: Analyzing 7Cu^{2+} Ion

The 7Cu^{2+} ion has the electron configuration [Ar]3d^9. With one unpaired electron in the d-orbital, it is paramagnetic and can show color because of d-d transitions.

Step 4: Analyzing 7Cu^{+} Ion

The 7Cu^{+} ion has the electron configuration [Ar]3d^{10}. With a fully filled d-orbital, it is not paramagnetic and ordinarily appears colorless since d-d transitions are not possible.

Step 5: Analyzing 7Ni^{2+} Ion

The 7Ni^{2+} ion has the electron configuration [Ar]3d^8. It has two unpaired electrons and is paramagnetic. It shows colors due to possible d-d transitions in its partially filled d-orbitals.

Step 6: Conclusion

Both 7Cu^{2+} and 7Ni^{2+} are paramagnetic and colored. Therefore, both exhibit these properties.

Key Concepts

Ion Paramagnetism

Paramagnetism in ions is a fascinating concept in transition metal chemistry.
It arises when ions have unpaired electrons in their electronic orbitals, specifically in their d-orbitals for transition metals. These unpaired electrons act as tiny magnets themselves, creating a net magnetic moment for the ion.
In simple terms, the more unpaired electrons an ion has, the stronger its paramagnetic properties.
In the context of the exercise, ions such as

• **\( \mathrm{Cu}^{2+} \) with a configuration of [Ar]3d\(^9\)** has one unpaired electron,
• **\( \mathrm{Ni}^{2+} \) with [Ar]3d\(^8\)** has two unpaired electrons.

Both demonstrate paramagnetism due to these unpaired electrons. On the other hand, \( \mathrm{Cu}^{+} \) has a fully filled d-orbital (3d\(^{10}\)), leaving no unpaired electrons, and thus is not paramagnetic. Understanding these characteristics of electron pairing helps in identifying the nature of ions in different chemical environments.

d-d Transitions

The term "d-d transitions" describes an electron's movement between d-orbitals of different energy levels.
This phenomenon occurs in transition metal compounds where d-orbitals split into different energy levels due to the presence of ligands. When an electron jumps from one d-orbital to another, it absorbs light energy.
The specific wavelength of this absorbed light is missing from the reflected light, and the color is perceived as the complementary color of the absorbed wavelength. This is why many transition metal ions appear colored.
For example, in the exercise,

• **\( \mathrm{Cu}^{2+} \)** ion's electron configuration [Ar]3d\(^9\) allows d-d transitions, imparting color.
• Similarly, **\( \mathrm{Ni}^{2+} \)** with its electron configuration [Ar]3d\(^8\) experiences d-d transitions, providing color.

Conversely, ions with completely filled or empty d-orbitals, like \( \mathrm{Cu}^{+} \), typically do not show this color characteristic, as d-d transitions are not feasible. Thus, understanding d-d transitions is essential for predicting the coloration in transition metal ions.

Electron Configuration

Electron configuration is a crucial concept in understanding the behavior of ions.
It denotes the distribution of electrons into various orbitals around an atom's nucleus. For transition metals, focus is often on their d-orbitals, as the arrangement of electrons here influences an ion’s properties.
The arrangement of electrons in these orbitals determines paramagnetism, coloration, and other chemical behaviors.For transition metal ions, the electron configuration is derived by removing electrons first from the s-orbital and then the d-orbitals during ion formation.

• Understanding the electron configurations of ions like**\( \mathrm{Cu}^{2+} \) ([Ar]3d\(^9\))** and **\( \mathrm{Ni}^{2+} \) ([Ar]3d\(^8\))** helps identify their paramagnetic nature and color.
• In contrast, **\( \mathrm{Cu}^{+} \)** with configuration [Ar]3d\(^{10}\) is significant as it does not exhibit paramagnetism or typical coloration due to absence of unpaired electrons and d-d transitions.

Mastering electron configurations provides insight into interpreting an ion's reactivity, magnetic property, and color, crucial for in-depth understanding of transition metal chemistry.