Problem 162 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(a) \(\mathrm{NH}_2^-\) - (p), (b) \(\mathrm{XeF}_4\) - (s), (c) \(\mathrm{CH}_3^-\) - (r), (d) \(\mathrm{NO}_2^-\) - (q).

Step 1: Understand the species in Column-I

Examine each chemical species in Column-I. These are ions or molecules with specific geometry and hybridization properties. For example, \(\mathrm{NH}_2^-\) typically has a bent shape due to the lone pair effects.

Step 2: Analyze potential matches from Column-II

Look at each option in Column-II. Each option describes a property or shape related to a species in Column-I. For example, (p) indicates a 'Bent shape', which may apply to \(\mathrm{NH}_2^-\).

Step 3: Match (a) \(\mathrm{NH}_2^-\)

\(\mathrm{NH}_2^-\) is an anion with one lone pair and two hydrogen atoms around nitrogen, leading to a bent shape due to lone pair-bond pair repulsion. Match with (p) Bent shape.

Step 4: Match (b) \(\mathrm{XeF}_4\)

\(\mathrm{XeF}_4\) has a central xenon atom surrounded by four fluorine atoms and two lone pairs, leading to a square planar shape. It matches with (s) square planar.

Step 5: Match (c) \(\mathrm{CH}_3^-\)

\(\mathrm{CH}_3^-\) is an anion with a carbon atom using \(\mathrm{sp}^3\) hybridization for three bonds and a lone pair, making (r) \(\mathrm{sp}^3\) hybridization the correct match.

Step 6: Match (d) \(\mathrm{NO}_2^-\)

In the \(\mathrm{NO}_2^-\) ion, nitrogen has one lone pair and forms an \(\mathrm{AB}_2\mathrm{E}_1\) arrangement leading to a bent shape. It matches with (q) \(\mathrm{AB}_2\mathrm{E}_2\).

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This geometry is determined by the number of bonds and lone pairs surrounding the central atom. For example, in the \(\mathrm{NH}_2^-\) ion, you'll notice it has a bent shape. This occurs because the nitrogen atom has two hydrogen atoms bonded to it and one lone pair. The lone pair exerts repulsion on the bonding pairs, resulting in a bent configuration.

On the other hand, \(\mathrm{XeF}_4\) showcases a square planar shape. In this molecule, the central xenon atom is surrounded by four fluorine atoms and two lone pairs. These lone pairs are positioned opposite each other, leading to a symmetrical square planar shape. Understanding these shapes is crucial as they influence the molecule's properties, like polarity and intermolecular interactions.

Hybridization

Hybridization is the concept of mixing atomic orbitals to create new hybrid orbitals suitable for bonding. This topic is essential for understanding how molecules form and retain their structure.

In the case of \(\mathrm{CH}_3^-\), the carbon atom undergoes \(\mathrm{sp}^3\) hybridization. This involves one s-orbital and three p-orbitals mixing to form four equivalent hybrid orbitals. Three of these orbitals create bonds with hydrogen, while the remaining orbital holds a lone pair.

Hybridization not only helps explain the molecular shape but also provides insights into the bond angles and the molecule's stability. For instance, \(\mathrm{sp}^3\) hybridization leads to a tetrahedral arrangement, with bond angles close to 109.5°, a feature that characterizes the \(\mathrm{CH}_3^-\) anion.

VSEPR Theory

VSEPR Theory, or Valence Shell Electron Pair Repulsion Theory, is used to predict the geometry of molecules based on repulsions between electron pairs in the valence shell of the central atom.

The key idea is that electron pairs—bonding and lone pairs—will arrange themselves to minimize repulsion. In \(\mathrm{NO}_2^-\), the VSEPR Theory predicts a bent shape due to the lone pair on nitrogen. This molecule adopts an \(\mathrm{AB}_2\mathrm{E}_1\) arrangement because the lone pair repels the bonded pairs into a less-than linear shape.

The utility of VSEPR Theory extends to complex geometries. It explains why \(\mathrm{XeF}_4\) with its “AX_4E_2” Electron Group Geometry ends up square planar; it’s due to electron pair repulsions and how lone pairs are positioned for minimal repulsion.