Problem 163 - A Complete Resource Book in Chemistry for JEE Main

Question

Match the following Column-I \(\quad\) Column-II (a) \(\mathrm{NH}_{2}^{-}\) (p) Bent shape (b) \(\mathrm{XeF}_{4}\) (q) \(\mathrm{AB}_{2} \mathrm{E}_{2}\) (c) \(\mathrm{CH}_{3}^{-}\) (r) \(\mathrm{sp}^{3}\) hybridization (d) \(\mathrm{NO}_{2}^{-}\) (s) square planar (t) lone pair

Step-by-Step Solution

Answer

(a) - (p), (b) - (s), (c) - (r), (d) - (q)

Step 1: Analyze NH2-

The species \( \mathrm{NH}_2^- \) has a bent shape due to the presence of a lone pair on nitrogen, which causes repulsion and bending the H-N-H bond angle. According to VSEPR theory, it follows \( \mathrm{AB}_2 \mathrm{E} \) geometry.

Step 2: Analyze XeF4

The molecule \( \mathrm{XeF}_4 \) has a square planar shape due to \( \mathrm{sp}^3\mathrm{d}^2 \) hybridization and involves four bonded pairs and two lone pairs on xenon, arranging them in a square planar form.

Step 3: Analyze CH3-

The molecule \( \mathrm{CH}_3^- \) has a tetrahedral shape and involves \( \mathrm{sp}^3 \) hybridization. The negative charge is due to an additional lone pair after forming three C-H bonds.

Step 4: Analyze NO2-

The ion \( \mathrm{NO}_2^- \) is bent due to resonance and \( \mathrm{sp}^2 \) hybridization. It follows the \( \mathrm{AB}_2 \mathrm{E}_2 \) geometry with one lone pair present.

Step 5: Create Matching

- \( \mathrm{NH}_2^- \) matches with (p) Bent shape.- \( \mathrm{XeF}_4 \) matches with (s) Square planar.- \( \mathrm{CH}_3^- \) matches with (r) \( \mathrm{sp}^3 \) hybridization.- \( \mathrm{NO}_2^- \) matches with (q) \( \mathrm{AB}_2 \mathrm{E}_2 \).

Key Concepts

Molecular Geometry

Molecular geometry is essential for understanding how atoms are arranged in a molecule, which influences its chemical properties and interactions. According to the VSEPR (Valence Shell Electron Pair Repulsion) theory, the shape of a molecule is dictated by the repulsion between electron pairs located around a central atom. This theory helps predict the arrangement of bonded atoms and lone pairs in a molecule:

• Bent shape: The \( \mathrm{NH}_2^- \) ion and \( \mathrm{NO}_2^- \) ion both exhibit a bent shape because lone pairs on the central atom cause the bonded pairs to move closer, allowing for optimal space and minimal repulsion.
• Square planar: In \( \mathrm{XeF}_4 \), the geometry is square planar. It's a result of the xenon atom forming a planar square shape with its four fluorine atoms, while the two lone pairs are positioned perpendicular to the plane, minimizing repulsion.

Understanding molecular geometry is crucial when predicting molecule behavior, reactivity, polarity, and interaction with other molecules.

Hybridization

Hybridization explains how atomic orbitals mix to form new hybrid orbitals. These hybrids aid in forming covalent bonds, dictating the overall shape and stability of a molecule. For instance:

• \( \mathrm{NH}_2^- \): Though not directly discussed in step-by-step solutions, it undergoes \( \mathrm{sp}^3 \) hybridization, resulting in a bent shape due to its three sigma bonds and one lone pair.
• \( \mathrm{CH}_3^- \): It showcases \( \mathrm{sp}^3 \) hybridization, with the carbon forming three bonds with hydrogen atoms, plus an extra lone pair making it tetrahedral.
• \( \mathrm{XeF}_4 \): This molecule uses \( \mathrm{sp}^3\mathrm{d}^2 \) hybridization, combining six orbitals to facilitate its square planar shape, accommodating bonds and lone pairs disordering.

Lone Pairs

Lone pairs are pairs of valence electrons that are not involved in bonding but significantly influence a molecule's shape and properties. They are typically located on a central atom and possess greater repulsion force than bonding pairs, causing shape alterations:

• Discordant forces: Lone pairs create additional repulsion compared to bonding pairs, leading to different molecular geometries to minimize repulsion.
• \( \mathrm{NH}_2^- \): The nitrogen atom harbors a lone pair; it pushes the hydrogen atoms closer to form a bent-shaped molecule.
• \( \mathrm{XeF}_4 \): The xenon atom contains two lone pairs, which remain unbonded but aligned so they occupy the most stable positions, ensuring the molecule remains square planar without distorting the shape.

Lone pairs play a critical role in determining the spatial design of molecules, influencing their dipole moments and physical characteristics.