Problem 94 - A Complete Resource Book in Chemistry for JEE Main

Question

Which among the following paramagnetic pairs rare? (1) \(\left[\mathrm{BaO}_{2}, \mathrm{NO}_{2}\right]\) (2) \(\left[\mathrm{KO}_{2}, \mathrm{NO}\right]\) (3) \(\left[\mathrm{H}_{2} \mathrm{O}_{2}, \mathrm{NO}\right]\) (4) \(\left[\mathrm{K}_{3} \mathrm{Fe}(\mathrm{CN})_{6}, \mathrm{CuCl}_{2}\right]\) (a) 3,4 only (b) 1,3 only (c) 2,4 only (d) \(1,2,4\)

Step-by-Step Solution

Answer

Option (c) 2,4.

Step 1: Understanding Paramagnetic Substances

Paramagnetic substances have unpaired electrons, which align with magnetic fields, making the substance attracted to magnets. To find the paramagnetic pairs, we need to determine which compounds in the pairs have unpaired electrons.

Step 2: Analyzing Option 1

In (1) In ext{BaO}_{2} Ba is in Ba2+ ( š[ ext{ns}^{2}], and šO2 is O O2- which has š[ ext{2 electrons} thus, it is not paramagnetic. Meanwhile, NO2 has an odd number of electrons (15), which results in unpaired electrons, making it paramagnetic.

Step 3: Analyzing Option 2

In (2), KO2 has a superoxide ion O2^- which possesses an unpaired electron, making it paramagnetic. NO has an odd number of electrons, causing an unpaired electron, making it also paramagnetic.

Step 4: Analyzing Option 3

In (3), H2O2 has all electrons paired and is thus diamagnetic, while NO is paramagnetic due to its unpaired electron.

Step 5: Analyzing Option 4

In (4), K3Fe( šCN )6 is a low-spin complex causing pairing in electrons, making it diamagnetic while retaining 18 electrons throughout. Meanwhile, CuCl2 typically has unpaired electrons in Cu2+, making it paramagnetic.

Step 6: Identifying Paramagnetic Pairs

From the above steps, we conclude: Option 1: NO2 is paramagnetic; BaO2 š is not. Option 2: Both KO2 and NO are paramagnetic. Option 3: Only NO is paramagnetic. Option 4: CuCl2 is paramagnetic.

Step 7: Choosing the Correct Answer

The pair with both compounds being paramagnetic is option 2. Therefore, from the given choices, option (c) is correct: 2,4.

Key Concepts

Unpaired Electrons

Unpaired electrons are crucial in understanding the behavior of paramagnetic substances. They are electrons in an atom that do not have a partner with opposite spin in the same orbital. This absence of pairing allows these electrons to align with external magnetic fields.
Upon interacting with a magnetic field, unpaired electrons generate a magnetic moment because they act as small magnets themselves. This is a hallmark of paramagnetism, where substances can be temporarily magnetized.
For instance, in the molecule \( NO_2 \), which has an odd number of electrons (15 in total), the unpaired electron contributes to its paramagnetic nature. Similarly, the molecule \( NO \), with 11 electrons, is paramagnetic because it also has an unpaired electron. These examples show that the presence of an unpaired electron is a predictor of whether a substance will exhibit paramagnetism.

Chemical Compounds

Chemical compounds are combinations of two or more elements that are bonded together in fixed proportions. They can form various types of substances, each with unique properties, such as paramagnetism, dependent on their atomic structure.
In the given exercise, understanding the electronic configuration of a compound is essential to determine its magnetic behavior. For example, \( BaO_2 \) consists of barium in the +2 oxidation state and peroxide anion \( O_2^{2-} \). However, its electronic configuration shows all electrons are paired, thus not paramagnetic.
Comparatively, \( KO_2 \), which contains the superoxide ion \( O_2^- \), contains an unpaired electron, making it paramagnetic. Similarly, compounds like \( CuCl_2 \) rely on the transition metal ion's electronic configuration, where \( Cu^{2+} \) has unpaired electrons, contributing to its paramagnetic nature. Understanding these principles helps determine the compositions and characteristics of chemical compounds.

Magnetic Properties

The magnetic properties of a substance are largely determined by the electronic structure of its atoms or molecules. Paramagnetism is one such property, where materials are attracted to an external magnetic field due to the presence of unpaired electrons.
To assess magnetic properties, it is essential to examine the electron arrangements in a compound. Materials classified as paramagnetic have unpaired electrons which create a net magnetic moment. As seen in \( KO_2 \), the superoxide anion contributes unpaired electrons, enhancing its magnetic attraction.
Alternatively, substances that have all electrons paired, like \( H_2O_2 \), are diamagnetic and often repel magnetic fields. Complex compounds, for instance in \( K_3Fe(CN)_6 \), may appear magnetic due to the transition metals but can be diamagnetic due to electron pairing at lower energy states in a low-spin state.

• Paramagnetism: Attraction to magnetic fields.
• Diamagnetism: Repulsion from magnetic fields.
• Ferromagnetism: Strong attraction, affecting more permanent magnetization.

These properties are fundamental in the application and understanding of chemical compounds in magnetic fields.