Problem 19 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not isostructural with \(\mathrm{SiCl}_{4} ?\) (a) \(\mathrm{PO}_{4}^{3-}\) (b) \(\mathrm{NH}_{4}^{+}\) (c) \(\mathrm{SCl}_{4}\) (d) \(\mathrm{SO}_{4}^{2-}\)

Step-by-Step Solution

Answer

\(\mathrm{SCl}_4\) is not isostructural with \(\mathrm{SiCl}_4\).

Step 1: Understand the Concept of Isostructural

Two chemical species are isostructural if they not only have the same number of atoms, but also share an identical geometric arrangement. In this question, we need to identify which option does not share the same geometry as \( \mathrm{SiCl}_4 \). \( \mathrm{SiCl}_4 \) has a tetrahedral shape.

Step 2: Determine Structure of \(\mathrm{SiCl}_4\)

\(\mathrm{SiCl}_4\) has a central silicon atom with four chlorine atoms bonded to it through single bonds, resulting in a tetrahedral shape. The silicon atom uses \(sp^3\) hybridization, which leads to this specific geometry.

Step 3: Analyze \(\mathrm{PO}_4^{3-}\)

\(\mathrm{PO}_4^{3-}\) has a phosphorus atom covalently bonded to four oxygen atoms, forming a tetrahedral shape. Like \(\mathrm{SiCl}_4\), it uses \(sp^3\) hybridization, making it isostructural with \(\mathrm{SiCl}_4\).

Step 4: Analyze \(\mathrm{NH}_4^{+}\)

In \(\mathrm{NH}_4^{+}\), the nitrogen is the central atom with four hydrogen atoms forming a tetrahedral shape due to \(sp^3\) hybridization. This is also isostructural with \(\mathrm{SiCl}_4\).

Step 5: Analyze \(\mathrm{SCl}_4\)

\(\mathrm{SCl}_4\) has a sulfur atom bonded to four chlorine atoms. The molecule has a see-saw or distorted tetrahedral structure due to the presence of a lone pair on the sulfur, which affects the symmetry and geometry, making it not isostructural with \(\mathrm{SiCl}_4\) which is a regular tetrahedron.

Step 6: Analyze \(\mathrm{SO}_4^{2-}\)

\(\mathrm{SO}_4^{2-}\) is formed by a sulfur atom with four oxygen atoms, resembling the tetrahedral shape due to \(sp^3\) hybridization. It retains a tetrahedral structure similar to \(\mathrm{SiCl}_4\), thus making it isostructural.

Step 7: Identify the Non-Isostructural Option

After analyzing all options, \(\mathrm{SCl}_4\) is the only molecule that does not exhibit a regular tetrahedral structure due to its lone pair. Therefore, \(\mathrm{SCl}_4\) is not isostructural with \(\mathrm{SiCl}_4\).

Key Concepts

Tetrahedral geometry

Tetrahedral geometry is a three-dimensional molecular shape that resembles a pyramid with a triangular base. This shape is easy to visualize if you picture a 'tripod' arrangement or a four-sided die used in board games.
One central atom is at the core, and four atoms (or groups of atoms) are positioned at each of the peaks, equidistant from the core.
A classic example of a molecule with tetrahedral geometry is \( \mathrm{SiCl}_4 \), where silicon is the central atom and the four chlorine atoms are symmetrically arranged around it.
The tetrahedral shape is a consequence of the central atom's electron pairs repelling each other equally, leading to an angle of approximately 109.5° between the bonds.
This geometry is crucial in understanding molecular structure and plays a significant role in determining the interactivity and function of molecules.

Molecular hybridization

Molecular hybridization involves the mixing or forming of new orbitals as atoms form compounds.
The concept of hybridization helps explain molecular geometries observed in chemical substances. For example, in tetrahedral molecules such as \( \mathrm{SiCl}_4 \), \( sp^3 \) hybridization occurs.
This means that one s orbital mixes with three p orbitals to create four equivalent hybrid orbitals.
These orbitals are oriented in a way that maximizes the distance between them, leading to the characteristic tetrahedral shape.

• \( \mathrm{PO}_4^{3-} \) and \( \mathrm{NH}_4^{+} \) undergo \( sp^3 \) hybridization, resulting in similar molecular shapes.
• Understanding hybridization allows predicting molecular shapes and the behavior of the molecule during chemical reactions.

It provides an insightful tool for anticipating the geometric orientation which molecules will likely favor.

Lone pair effects

Lone pairs are pairs of electrons on an atom that are not shared with other atoms and remain nonbonding. They have significant effects on the geometry of molecules.
Lone pairs occupy more space than bonding pairs since their electron clouds aren't restricted between two nuclei.
This can lead to angular distortion in a molecule's geometry, influencing its overall shape.
For example, in \( \mathrm{SCl}_4 \), the presence of a lone pair on sulfur results in a see-saw shape rather than a regular tetrahedral geometry. This lone pair repels bonding electron pairs, compressing bond angles and creating asymmetry.

• The theory of lone pairs helps explain why two compounds with similar hybridizations can have different shapes.
• These effects are key considerations in predicting structural changes and reactivity issues in chemistry.

Understanding lone pair effects deepens comprehension of molecular configurations beyond basic atomic arrangements.