Problem 25 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In which of the following ionization processes the bond energy has increased and also the magnetic behaviour has changed from paramagnetic to diamagnetic ? [Main Online April 9, 2013] (a) \(\mathrm{NO} \rightarrow \mathrm{NO}^{+}\) (b) \(\mathrm{N}_{2} \rightarrow \mathrm{N}_{2}^{+}\) (c) \(\mathrm{C}_{2} \rightarrow \mathrm{C}_{2}{\underline{\phantom{xx}}}^{+}\) (d) \(\mathrm{O}_{2} \rightarrow \mathrm{O}_{2}^{+}\)

Step-by-Step Solution

Answer

Option (a) - \\mathrm{NO} \\rightarrow \\mathrm{NO}^{+} satisfies both conditions.

Step 1: Understanding the Context

In ionization processes where a neutral molecule loses an electron to become a cation, two key aspects often change: bond energy and magnetic behavior. When evaluating these changes, note that losing an unpaired electron can affect whether a molecule remains paramagnetic (unpaired electrons) or becomes diamagnetic (only paired electrons). Typically, bond energy increases when a more stable electronic configuration is achieved.

Step 2: Analyze Magnetic Behavior

Each molecule starts with a given magnetic property: paramagnetic (unpaired electrons) or diamagnetic (paired electrons). We need to check if the ionization changes the number of unpaired electrons. (a) NO is initially paramagnetic (1 unpaired electron). (b) N₂ is diamagnetic (all electrons paired). (c) C₂ is diamagnetic (all electrons paired). (d) O₂ is paramagnetic (2 unpaired electrons). If after ionization, the molecule ends up with paired electrons, it becomes diamagnetic.

Step 3: Determine Bond Energy Change

Ionization can either stabilize or destabilize a molecule, typically measured by bond energy. Removing an electron from antibonding or bonding molecular orbitals can determine this. For bond energy: (a) NO to NO⁺ increases because the removed electron is from an antibonding orbital. (b) N₂ to N₂⁺ decreases because the removed electron destabilizes the bond. (c) C₂ to C₂⁺ decreases slightly, minimal change to bond orbitals. (d) O₂ to O₂⁺ increases because removing one electron from the antibonding orbital strengthens the bond.

Step 4: Evaluate Changes

According to our findings: (a) NO to NO⁺: Changes from paramagnetic to diamagnetic, and bond energy increases. (b) N₂ to N₂⁺: Bond energy decreases, no magnetic change. (c) C₂ to C₂⁺: Bond energy decreases slightly, no magnetic change. (d) O₂ to O₂⁺: Increases bond energy, but magnetic behavior only changes to single unpaired electron (still paramagnetic). Thus, only (a) satisfies both conditions.

Key Concepts

Ionization Processes

Ionization processes involve the removal of an electron from a molecule, converting it into a cation. This change can significantly affect the molecule's properties. When a neutral molecule becomes ionized, the electronic configuration alters, which may lead to changes in bond strength and magnetic behavior. An important aspect to consider is where the electron is removed from – whether it is from a bonding or antibonding molecular orbital. This determines if the bond energy will increase or decrease.

• If an electron is removed from a bonding orbital, the bond weakens.
• If an electron is removed from an antibonding orbital, the bond strengthens.

However, for a molecule to stabilize after ionization, achieving a more stable electronic arrangement is key, often signaled by the increase in bond energy.

Magnetic Behaviour

Magnetic behavior of molecules is dictated by the presence of unpaired electrons in their molecular orbitals.

• 'Paramagnetic' molecules have unpaired electrons and are attracted by magnetic fields.
• 'Diamagnetic' molecules have all electrons paired and are slightly repelled by magnetic fields.

During ionization, the loss of an unpaired electron can change the molecule from being paramagnetic to diamagnetic. This simple transition is key in phenomena like how materials interact with magnetic fields. Evaluating these changes requires understanding the original electron configuration, and observing how it shifts post-ionization. In our analysis, if a paramagnetic molecule loses its unpaired electron, it can turn into a diamagnetic species, reflecting a change in its magnetic behavior.

Bond Energy

Bond energy is a crucial factor in determining the stability of a molecule. It refers to the energy required to break a bond between atoms. When ionization occurs, examining whether the electron is lost from a bonding or antibonding orbital helps predict the bond energy change.

• Removing an electron from an antibonding orbital often increases bond energy because it reduces the overall electron repulsion.
• Conversely, if removed from a bonding orbital, bond energy may decrease, destabilizing the molecule.

Therefore, the case with \(\mathrm{NO} \rightarrow \mathrm{NO}^{+}\), where the removal of an electron from an antibonding orbital leads to an increase in bond energy, represents an example where ionization brings about a "stronger" molecule. Recognizing these changes is essential in fields such as chemical bonding and molecular stability analysis.

Paramagnetism and Diamagnetism

Paramagnetism and diamagnetism are fundamental magnetic properties of molecules.

• Paramagnetic molecules contain one or more unpaired electrons, making them attracted to magnetic fields. They play a crucial role in fields like magnetic resonance imaging (MRI).
• Diamagnetic molecules have all their electrons paired. They exhibit a weak repulsion to magnetic fields, evident in everyday materials like copper and water.

The transition from paramagnetic to diamagnetic behavior during ionization, as seen when a molecule like \(\mathrm{NO}\) becomes \(\mathrm{NO}^{+}\), highlights an instance of changing magnetic properties. Understanding which electron is lost during ionization allows for predicting the magnetic shift, crucial in the design of electronic and magnetic materials.