Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following species, identify the isostructural pairs \(\mathrm{NF}_{3}, \mathrm{NO}_{3}^{-}, \mathrm{BF}_{3}, \mathrm{H}_{3} \mathrm{O}^{+}, \mathrm{HN}_{3}\) (a) \(\left[\mathrm{NF}_{3}, \mathrm{NO}_{3}^{-}\right]\)and \(\left[\mathrm{BF}_{3}, \mathrm{H}_{3} \mathrm{O}^{+}\right]\) (b) \(\left[\mathrm{NF}_{3}, \mathrm{HN}_{3}\right]\) and \(\left[\mathrm{NO}_{3}-\mathrm{BF}_{3}\right]\) (c) \(\left[\mathrm{NF}_{3}, \mathrm{H}_{3} \mathrm{O}^{+}\right]\)and \(\left[\mathrm{NO}_{3}-\mathrm{BF}_{3}\right]\) (d) \(\left[\mathrm{NF}_{3}, \mathrm{H}_{3} \mathrm{O}^{+}\right]\)and \(\left[\mathrm{HN}_{3}, \mathrm{BF}_{3}\right]\)

Step-by-Step Solution

Answer

Correct isostructural pairs: (c) NF3 & H3O+ (trigonal pyramidal); NO3^- & BF3 (trigonal planar).

Step 1: Understanding Isostructural Species

Isostructural species have the same molecular geometry or shape. To identify them, we need to determine the molecular geometry of each given species.

Step 2: Determine NF3 Geometry

NF3 has a nitrogen center with three fluorine atoms and one lone pair of electrons. It adopts a trigonal pyramidal shape due to the lone pair on nitrogen, distorting the basic tetrahedral geometry.

Step 3: Determine NO3^- Geometry

NO3^- has three oxygen atoms attached to the central nitrogen atom and no lone pairs, resulting in a trigonal planar geometry. Its shape is flat with 120° bond angles.

Step 4: Determine BF3 Geometry

BF3, like NO3^-, is trigonal planar. It has a boron atom at the center connected to three fluorine atoms, creating a flat molecule with 120° bond angles.

Step 5: Determine H3O+ Geometry

H3O+ has a central oxygen atom with three hydrogen atoms bonded and one lone pair of electrons. This gives it a trigonal pyramidal shape similar to NF3.

Step 6: Determine HN3 Geometry

HN3 has a linear molecular geometry as it is essentially a chain of nitrogen atoms bonded together with one hydrogen atom.

Step 7: Compare Geometries

By comparing geometries, we identify which pairs have the same shape: NF3 and H3O+ are both trigonal pyramidal, while NO3^- and BF3 are both trigonal planar.

Key Concepts

Molecular Geometry

Molecular geometry is the three-dimensional arrangement of atoms within a molecule. It is a crucial concept that helps in understanding the behavior, reactivity, and properties of molecules. Molecular geometry is determined by factors such as the number of atoms bonded to the central atom, the number of lone pairs on the central atom, and the electron-pair repulsion, often explained by VSEPR (Valence Shell Electron Pair Repulsion) theory.

Understanding molecular geometry is fundamental when identifying isostructural species. Isostructural species have identical molecular shapes, leading to similar chemical properties. Determining if two species are isostructural involves analyzing the geometry which can vary based on the presence of lone pairs and types of atoms within the molecule.

Trigonal Pyramidal

A trigonal pyramidal molecular geometry occurs when there is a central atom bonded to three other atoms with a lone pair of electrons. This geometry resembles a pyramid with a triangular base. The lone pair on the central atom exerts repulsion on the bonded atoms, leading to this specific shape.

A classic example of a trigonal pyramidal molecule is \( ext{NF}_3\). The nitrogen atom is the central atom, bonded to three fluorine atoms. The lone pair on nitrogen causes repulsion that bends the geometry away from a simple planar shape. Similarly, the hydronium ion, \( ext{H}_3 ext{O}^+\), also exhibits trigonal pyramidal geometry due to its central oxygen atom bonded to three hydrogens, with a lone pair exerting repulsion.

This geometry is what makes \( ext{NF}_3\) and \( ext{H}_3 ext{O}^+\) isostructural. Both follow the trigonal pyramidal structure, having a lone pair on the central atom which influences their three-dimensional shape.

Trigonal Planar

Trigonal planar is a type of molecular geometry where the central atom is connected to three atoms, situated at the corners of an equilateral triangle. This geometry results in a flat, two-dimensional shape with 120° bond angles. Unlike the trigonal pyramidal shape, the trigonal planar shape has no lone pair influence, resulting in a symmetrical and even distribution.

\( ext{BF}_3\) is a common example of a molecule that features trigonal planar geometry. The central boron atom is bonded to three fluorine atoms in a plane, leading to a structure that is uniform and stable without any lone pair repulsing the bond angles. \( ext{NO}_3^-\), the nitrate ion, also displays a trigonal planar shape, where the central nitrogen is bonded to three oxygen atoms, maintaining the planar arrangement.

Due to their identical trigonal planar geometries, \( ext{NO}_3^-\) and \( ext{BF}_3\) are considered isostructural species. Their planar shape and the distribution of atoms leads to similar chemical properties despite the difference in chemical composition.

Lone Pair Effect

Lone pairs play a significant role in determining the molecular geometry of a compound. These are pairs of valence electrons not shared with another atom. Due to their negative charge, lone pairs tend to repel the bonded pairs of electrons, affecting the overall shape of the molecule.

The lone pair effect is a critical factor in differentiating trigonal pyramidal and trigonal planar geometries. For instance, in \( ext{NF}_3\), the lone pair on nitrogen not involved in bonding causes a deviation from a planar arrangement, resulting in a trigonal pyramidal shape. Similarly, in \( ext{H}_3 ext{O}^+\), the lone pair on oxygen creates a repulsion, pushing the hydrogen atoms into a pyramidal shape.

In contrast, \( ext{BF}_3\) lacks lone pairs around the central boron atom, allowing the bonded fluorines to distribute evenly around the center, forming a trigonal planar geometry. Understanding the implications of lone pair repulsions is essential for predicting molecular shapes and identifying isostructural partners within chemical systems.