Problem 149 - A Complete Resource Book in Chemistry for JEE Main

Question

Isostructural group of molecule is (a) \(\mathrm{XeO}_{4}, \mathrm{NH}_{4}^{+}, \mathrm{CH}_{4}\) (b) \(\mathrm{CH}_{3}^{-}, \mathrm{NH}_{3}, \mathrm{NF}_{3}\) (c) \(\mathrm{NH}_{3}, \mathrm{NF}_{3}, \mathrm{BF}_{3}\) (d) \(\mathrm{NO}_{3}, \mathrm{NO}_{2}, \mathrm{SF}_{4}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{XeO}_{4}, \mathrm{NH}_{4}^{+}, \mathrm{CH}_{4}\) are isostructural.

Step 1: Understand Isostructural Molecules

Isostructural molecules have the same shape and symmetry but can contain different atoms. They typically have the same number of bonding pairs and lone electron pairs on the central atom.

Step 2: Identify VSEPR Shapes of Molecules

Determine the VSEPR shapes for each molecule or ion in the given options.- (a) - \(\mathrm{XeO}_{4}\) is tetrahedral, - \(\mathrm{NH}_{4}^{+}\) is tetrahedral, - \(\mathrm{CH}_{4}\) is tetrahedral.(b) - \(\mathrm{CH}_{3}^{-}\) is trigonal pyramidal,- \(\mathrm{NH}_{3}\) is also trigonal pyramidal,- \(\mathrm{NF}_{3}\) is also trigonal pyramidal.(c) - \(\mathrm{NH}_{3}\) is trigonal pyramidal,- \(\mathrm{NF}_{3}\) is trigonal pyramidal,- \(\mathrm{BF}_{3}\) is trigonal planar.(d) - \(\mathrm{NO}_{3}^{-}\) is trigonal planar,- \(\mathrm{NO}_{2}\) is bent,- \(\mathrm{SF}_{4}\) is seesaw-shaped.

Step 3: Compare Shapes and Structures

Compare the geometries of the molecules in each set of the given options: - For option (a), all molecules are tetrahedral. - For option (b), all molecules are trigonal pyramidal. - For option (c), two molecules are trigonal pyramidal and one is trigonal planar. - For option (d), all have different shapes: trigonal planar, bent, and seesaw.

Step 4: Choose the Isostructural Group

The isostructural group is the one where all species have the same geometry. Compare the geometries from Step 3 and identify the matching group. You see that both (a) and (b) have consistent shapes across their molecules, but they have different geometries, where (a) is tetrahedral and (b) is trigonal pyramidal.

Key Concepts

VSEPR Theory

VSEPR stands for Valence Shell Electron Pair Repulsion theory. It is a model used to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms. The key idea is that electron pairs, whether bonding or non-bonding, will arrange themselves as far apart as possible around the central atom to minimize repulsion.

• Electron pairs include both bonding pairs and lone pairs.
• The geometrical arrangement of these electron pairs dictates the overall shape of the molecule.

VSEPR theory allows us to predict the shape of a molecule simply by knowing the number of bonds and lone pairs around the central atom. This is crucial in determining physical and chemical properties of molecules, such as polarity and reactivity.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This structure is determined by the distances and angles between atoms, which are influenced by electron pair repulsion, as explained by VSEPR theory. Understanding molecular geometry helps in predicting and explaining the behavior and reactivity of molecules.

• The geometry affects properties like polarity and intermolecular forces.
• Diverse configurations include linear, bent, tetrahedral, and more.

Geometries such as tetrahedral and trigonal pyramidal play a significant role in defining how molecules interact with each other and with external fields, serving as the backbone for understanding advanced chemical interactions.

Tetrahedral Geometry

Tetrahedral geometry is one of the most common molecular shapes and occurs when four pairs of electrons are placed around a central atom, creating an angle of about 109.5° between them. This formation is characteristic of molecules like \(\mathrm{CH}_{4}\) and \(\mathrm{NH}_{4}^{+}\).

• A tetrahedral molecule has four bonds arranged symmetrically around the central atom.
• It is highly symmetric and typically does not have lone pairs on the central atom.

The symmetry associated with tetrahedral geometry often leads to non-polar molecules unless different atoms are attached to the central atom, which can induce a dipole moment. This arrangement maximizes the distance between electron pairs, minimizing repulsion and creating a stable structure.

Trigonal Pyramidal Geometry

Trigonal pyramidal geometry occurs when a molecule has three bonding pairs and one lone pair on the central atom. This lone pair exerts a greater repulsive force on the bonding pairs, which results in a shape different from the ideal triangular base. An example of a trigonal pyramidal molecule includes \(\mathrm{NH}_{3}\).

• The bond angles are slightly less than the ideal 109.5° found in a perfect tetrahedron.
• The central atom is located at the apex of the pyramid formed by the three atoms at the base.

This geometry is less symmetrical compared to tetrahedral structures. The presence of the lone pair at the apex causes distortion, which can result in polar molecules. In contrast to a tetrahedral molecule, the lone pair in a trigonal pyramidal shape affects the molecular polarity and its interactions with other molecules.