Problem 203 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following ionization processes, the bond order has increased and the magnetic behaviour has changed? (a) \(\mathrm{NO} \rightarrow \mathrm{NO}^{+}\) (b) \(\mathrm{O}_{2} \rightarrow \mathrm{O}_{2}^{+}\) (c) \(\mathrm{N}_{2} \rightarrow \mathrm{N}_{2}^{+}\) (d) \(\mathrm{C}_{2} \rightarrow \mathrm{C}_{2}^{+}\)

Step-by-Step Solution

Answer

(a) NO → NO⁺ The bond order increases, and the magnetic behavior changes from paramagnetic to diamagnetic.

Step 1: Understanding Bond Order

The bond order is calculated using the formula \(\frac{(\text{Number of bonding electrons} - \text{Number of antibonding electrons})}{2}\). A higher bond order means a stronger bond.

Step 2: Analyzing the Bond Order for NO and NO⁺

NO has 11 valence electrons, with 8 in bonding orbitals and 3 in antibonding, so the bond order is \((8 - 3)/2 = 2.5\). NO⁺ has 10 electrons, with 8 in bonding orbitals and 2 in antibonding, so the bond order is \((8 - 2)/2 = 3\). The bond order increases in this process.

Step 3: Analyzing the Bond Order for O2 and O2⁺

O2 has 12 valence electrons, with 8 in bonding orbitals and 4 in antibonding, so the bond order is \((8 - 4)/2 = 2\). O2⁺ has 11 electrons, with 8 in bonding orbitals and 3 in antibonding, so the bond order is \((8 - 3)/2 = 2.5\). The bond order increases in this process.

Step 4: Analyzing the Bond Order for N2 and N2⁺

N2 has 10 valence electrons, with 8 in bonding orbitals and 2 in antibonding, so the bond order is \((8 - 2)/2 = 3\). N2⁺ has 9 electrons, with 8 in bonding orbitals and 1 in antibonding, so the bond order is \((8 - 1)/2 = 3.5\). The bond order increases in this process.

Step 5: Analyzing the Bond Order for C2 and C2⁺

C2 has 8 valence electrons, all in bonding orbitals, so the bond order is \((8 - 0)/2 = 4\). C2⁺ has 7 electrons, with 8 in bonding orbitals and 1 in antibonding, so the bond order is \((8 - 1)/2 = 3.5\). The bond order decreases in this process.

Step 6: Examining Magnetic Behaviour Changes

NO is paramagnetic due to its one unpaired electron, while NO⁺ becomes diamagnetic as there are no unpaired electrons. O2 is paramagnetic, and O2⁺ remains paramagnetic with one unpaired electron. N2 is diamagnetic, and N2⁺ becomes paramagnetic with one unpaired electron. C2 is diamagnetic, and C2⁺ remains diamagnetic.

Key Concepts

Bond Order

The concept of bond order is a crucial one in the study of molecular orbitals. It reflects the strength and stability of a bond between two atoms. Bond order is calculated using the formula: \[ \text{Bond Order} = \frac{\text{Number of bonding electrons - Number of antibonding electrons}}{2} \] A higher bond order indicates a stronger and more stable bond, as more electrons are involved in holding the two atoms together.

• A bond order of 1 signifies a single bond, where electrons are participating in creating a single pairing.
• A bond order of 2 indicates a double bond, reflecting a stronger interaction.
• Similarly, a bond order of 3 suggests a triple bond, the strongest type.

Understanding the change in bond order during ionization or other chemical processes can help predict the stability of the resulting molecule.For example, in the ionization of nitric oxide (\(\text{NO} \rightarrow\text{NO}^+\)), the bond order increases from 2.5 to 3, suggesting a stronger bond post-ionization.

Paramagnetism and Diamagnetism

Paramagnetism and diamagnetism describe the magnetic properties of substances, heavily influenced by the presence of unpaired electrons in molecular orbitals.

• A molecule is termed **paramagnetic** if it has one or more unpaired electrons. These unpaired electrons cause the molecule to be attracted by external magnetic fields.
• **Diamagnetic** substances, on the other hand, have all electrons paired. These substances are slightly repelled by a magnetic field.

The change in a molecule's magnetic properties can be influenced by ionization processes that alter the number of unpaired electrons. For instance, nitrogen monoxide (\(\text{NO}\)) is paramagnetic due to its unpaired electron, but \(\text{NO}^+\) becomes diamagnetic as the ionization process removes this unpaired electron. Conversely, nitrogen molecule (\(\text{N}_2\)) is diamagnetic, but turns into a paramagnetic \(\text{N}_2^+\) ion upon ionization, as an unpaired electron is introduced.

Ionization Processes

Ionization is the process in which an atom or a molecule loses or gains an electron, transforming into an ion. This change impacts various properties of the molecule, including bond order and magnetic behavior. In the context of molecular orbitals:

• **Bond order** can either increase or decrease upon ionization, depending on whether electrons are removed from bonding or antibonding orbitals.
• **Magnetic behavior** can change if the process alters the number of unpaired electrons.

For example, when \(\text{O}_2\) becomes \(\text{O}_2^+\), an electron is removed from an antibonding orbital, increasing the bond order and maintaining its paramagnetic nature.Understanding ionization processes is essential for predicting how a molecule will behave chemically and physically after gaining or losing electrons.