Problem 54 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following shows isostructural species? (a) \(\mathrm{NH}_{4}^{+}\)and \(\mathrm{NH}_{2}^{-}\) (b) \(\mathrm{CH}_{3}^{-}\)and \(\mathrm{CH}_{3}^{+}\) (c) \(\mathrm{SO}_{4}^{2-}, \mathrm{PO}_{4}^{3-}\) and \(\left[\mathrm{BF}_{4}-\right]\) (d) \(\mathrm{NH}_{4}^{+}\)and \(\mathrm{NH}_{3}\)

Step-by-Step Solution

Answer

Option (c) shows isostructural species.

Step 1: Understanding Isostructural Species

Two species are isostructural if they have the same number of atoms and the same structure, including the same hybridization of the central atom and a similar or homologous spatial arrangement of atoms.

Step 2: Analyzing Option (a)

The species \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{NH}_{2}^{-}\) have different numbers of surrounding atoms and different hybridizations ( 2sp^3 and sp^2, respectively). Therefore, they are not isostructural.

Step 3: Analyzing Option (b)

The species \(\mathrm{CH}_{3}^{-}\) and \(\mathrm{CH}_{3}^{+}\) also have different hybridizations. \(\mathrm{CH}_{3}^{-}\) is 2sp^3, whereas \(\mathrm{CH}_{3}^{+}\) is 2sp^2, so they are not isostructural.

Step 4: Analyzing Option (c)

\(\mathrm{SO}_{4}^{2-}\), \(\mathrm{PO}_{4}^{3-}\), and \(\left[\mathrm{BF}_{4}-\right]\) all have a tetrahedral structure with 2sp^3 hybridization. Therefore, these species are isostructural.

Step 5: Analyzing Option (d)

The species \(\mathrm{NH}_{4}^{+}\) and \(\mathrm{NH}_{3}\) have different numbers of hydrogen atoms and different geometries (tetrahedral vs. trigonal pyramidal). Thus, they are not isostructural.

Step 6: Conclusion

The species in option (c) are isostructural because they all share the same tetrahedral structure and hybridization.

Key Concepts

Hybridization

Hybridization is a crucial concept in understanding chemical bonding and molecular shapes. It refers to the mixing of atomic orbitals to form new hybrid orbitals suitable for the pairing of electrons to form chemical bonds. This usually involves combining different types of orbitals, such as s and p orbitals, from the same atom into new identical orbitals that are oriented in a particular geometric shape.

For example, in a molecule like methane ( CH_4 ), the carbon atom undergoes sp^3 hybridization. This means that one s orbital and three p orbitals mix, resulting in four equivalent sp^3 hybrid orbitals that form a tetrahedral shape when bonding with hydrogen.

Hybridization helps explain the shape and bonding of complex molecules because it provides a more accurate picture than using traditional atomic orbitals alone. Understanding hybridization allows us to predict molecular shapes and the arrangement of atoms, which is fundamental for identifying isostructural species.

Tetrahedral Structure

A tetrahedral structure occurs when a central atom is bonded to four other atoms, positioned at the corners of a tetrahedron. This geometric shape ensures that the atoms are as far apart from each other as possible to minimize repulsion, following VSEPR theory (Valence Shell Electron Pair Repulsion theory).

In a tetrahedral structure, bond angles are approximately 109.5 degrees, and this specific arrangement is often found in molecules with sp^3 hybridization. Examples include the NH_4^+ ion and methane ( CH_4 ).

Understanding the tetrahedral arrangement is essential for recognizing molecules that are isostructural. Since isostructural species share the same geometry and hybridization, identifying whether a molecule depicts a tetrahedral structure can help you determine similarities with other species.

Chemical Species

Chemical species refer to atoms, molecules, ions, or radicals that have the same characteristic chemical properties. When discussing isostructural species, chemical species must share both their structural geometry and hybridization level.

Examples of chemical species include the sulfate ion ( SO_4^{2-} ), phosphate ion ( PO_4^{3-} ), and the tetrafluoroborate ion ( BF_4^{-} ). These specific species all have a tetrahedral configuration with sp^3 hybridization, qualifying them as isostructural.

Understanding chemical species in terms of their structural shapes and hybridization allows chemists to anticipate their properties and reactivities. It helps categorize and compare seemingly diverse molecules that might have common structural or functional characteristics, aiding in fields like chemical synthesis, reaction prediction, and material science.