Problem 193 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has the regular tetrahedral structure? (a) \(\mathrm{BF}_{4}{\underline{\phantom{xx}}}^{-}\) (b) \(\mathrm{SF}_{4}\) (c) \(\left[\mathrm{Ni}(\mathrm{CN})_{a}\right]^{2-}\) (d) \(\mathrm{XeF}_{4}\)

Step-by-Step Solution

Answer

Option (a) \\(\mathrm{BF}_4^-\\) has a regular tetrahedral structure.

Step 1: Understanding the Question

We need to determine which of the given chemical species has a regular tetrahedral structure. A regular tetrahedral structure has a central atom with four identical atoms bonded to it, and the bond angles are approximately 109.5°.

Step 2: Analyzing Option (a)

The species is \(\mathrm{BF}_4^-\). Boron typically forms three bonds, but in \(\mathrm{BF}_4^-\), boron forms four bonds with fluorine atoms, resulting in a negative charge. These bonds are equidistant, leading to a regular tetrahedral structure with bond angles of 109.5°.

Step 3: Analyzing Option (b)

The species is \(\mathrm{SF}_4\). Sulfur in \(\mathrm{SF}_4\) forms four single bonds with fluorine atoms and has one lone pair. This results in a see-saw molecular geometry, not a regular tetrahedral structure.

Step 4: Analyzing Option (c)

The species is \(\left[\mathrm{Ni}(\mathrm{CN})_4\right]^{2-}\). Nickel here is coordinated in a complex with cyanide ions. The common geometry for such complexes is square planar, not tetrahedral.

Step 5: Analyzing Option (d)

The species is \(\mathrm{XeF}_4\). Xenon in \(\mathrm{XeF}_4\) forms four bonds with fluorine and has two lone pairs. This leads to a square planar geometry, not a tetrahedral one.

Step 6: Conclusion

Based on the analysis, option (a) \(\mathrm{BF}_4^-\) has a regular tetrahedral structure due to its bonding configuration.

Key Concepts

Molecular Geometry

When thinking about molecular geometry, visualize the 3D shape of a molecule. The arrangement of atoms in a compound dictates its shape. This shape influences everything from reactivity to polarity and even biological activity.

In a regular tetrahedral structure, which is a key concept, a central atom is symmetrically surrounded by four other atoms. Think of it like a pyramid with a triangular base. All bond angles in a tetrahedral molecule are close to 109.5°, creating an equilibrium that balances repulsive forces between electron pairs. This structure is common for many molecules and ions, facilitating equal distribution of bond stress and maximizing the spatial separation between bonded pairs.

• Central atom is bonded to four identical atoms.
• Bond angles are approximately 109.5°.
• Symmetry prevents the molecule from having dipole moments (it's nonpolar).

Molecules, like \(\mathrm{BF}_4^-\), illustrate this perfectly as each fluorine atom positions itself equidistantly relative to the others.

Lewis Structure

Lewis structures are like blueprints for molecules. They show which atoms are connected and how, using lines to represent shared pairs of electrons (bonds). This visual can also reveal lone pairs of electrons, which are not shared between two atoms but are still significant as they affect the shape and properties of the molecule.

Creating a Lewis structure involves several steps:

• Count the total valence electrons available for bonding.
• Identify the central atom (usually the least electronegative).
• Form bonds between atoms, allocating electrons to achieve full octets for most atoms.
• Place any remaining electrons as lone pairs.

Understanding these basics helps clarify why options like \(\mathrm{SF}_4\) and \(\mathrm{XeF}_4\) possess different geometries. For instance, lone pairs in \(\mathrm{SF}_4\) result in a see-saw shape due to increased repulsion compared to bonded pairs. Thus, Lewis structures are foundational in predicting the possible 3D configurations of molecules.

Chemical Bonding

At its core, chemical bonding involves the joining of atoms to form molecules or compounds. Bonds come primarily in two forms, covalent and ionic, with each type having unique characteristics and effects on the resulting molecular structure.

Covalent bonds are formed by the sharing of electrons between atoms, leading to the creation of distinct molecules. These can be single, double, or triple bonds, depending on the number of shared electron pairs. This sharing allows atoms to satisfy their octet or duet rule, becoming more stable, as depicted in \(\mathrm{BF}_4^-\) where boron shares electrons with fluorine.

• Covalent bonding: Involves shared electron pairs.
• Ionic bonding: Involves the transfer of electrons, resulting in charged ions.
• Metallic bonding: Involves a sea of shared electrons among metal atoms.

Ionic bonding contrasts with covalent, where electron transfer between atoms leads to positively and negatively charged ions. Such differences in electron interaction play a pivotal role in determining a substance's physical properties and reactivity.