Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following molecules all the bonds are not equal? (a) \(\mathrm{AlF}_{3}\) (b) \(\mathrm{NF}_{3}\) (c) \(\mathrm{ClF}_{3}\) (d) \(\mathrm{BF}_{3}\)

Step-by-Step Solution

Answer

The bonds are not all equal in \(\mathrm{ClF}_3\).

Step 1: Analyze Molecule AlF_3

The molecule \(\mathrm{AlF}_3\) consists of aluminum (Al) bonded to three fluorine (F) atoms. Aluminum forms a symmetrical structure, resulting in all Al-F bonds being equal. Therefore, all bonds in \(\mathrm{AlF}_3\) are equal.

Step 2: Analyze Molecule NF_3

The molecule \(\mathrm{NF}_3\) has a nitrogen (N) atom bonded to three fluorine (F) atoms. This forms a trigonal pyramidal structure, and since all bond pairs involve N and F, all N-F bonds are equal in length and strength.

Step 3: Analyze Molecule ClF_3

The molecule \(\mathrm{ClF}_3\) has a chlorine (Cl) atom bonded to three fluorine (F) atoms. It has a T-shaped structure with three equatorial positions occupied by fluorine atoms, but due to lone pair-bond pair interactions, not all Cl-F bonds are equal in length and strength.

Step 4: Analyze Molecule BF_3

The molecule \(\mathrm{BF}_3\) is composed of boron (B) bonded to three fluorine (F) atoms in a trigonal planar structure. This leads to all B-F bonds being equal in length and strength due to the symmetry of the molecule.

Step 5: Conclusion

Among the molecules \(\mathrm{AlF}_3\), \(\mathrm{NF}_3\), \(\mathrm{ClF}_3\), and \(\mathrm{BF}_3\), the one where all bonds are not equal is \(\mathrm{ClF}_3\). This is due to the T-shaped molecular geometry causing variation in bond lengths.

Key Concepts

Chemical Bonding

At the heart of molecular geometry is chemical bonding. It's the process where atoms connect to form molecules. These bonds are the "glue," holding atoms together and determining the shapes, lengths, and angles between them.
The two most common types of chemical bonds are ionic and covalent.

• Ionic Bonds: Formed when electrons are transferred between atoms, leading to attraction between charged ions.
• Covalent Bonds: Occur when atoms share electron pairs, providing stability.

The nature of the bond (ionic or covalent) largely dictates how atoms are arranged and how the molecule behaves.
Understanding these bond types and formations is crucial since it sets the stage for how molecules like \( \mathrm{ClF}_{3} \) and \( \mathrm{BF}_{3} \) take their form.

Symmetrical Structures

Symmetrical structures in molecules occur when atoms are arranged evenly, resulting in equal bond lengths and angles. This symmetry can convey stability and a balance of electrical charges within the molecule.
For example, \( \mathrm{AlF}_3 \) and \( \mathrm{BF}_3 \) exhibit high symmetry that leads to equal bond lengths.
Key features of symmetrical structures include:

• Uniform arrangement of atoms around a central atom.
• Equal bond angles between atoms.
• Often result in nonpolar molecules when external forces are balanced equally.

In such structures, the distribution of shared electron pairs is even, resulting in geometrical shapes such as linear, planar, or tetrahedral.
This symmetry plays a pivotal role in determining molecular polarity and chemical behavior.

T-shaped Structure

The T-shaped structure is a less common molecular geometry resulting from specific arrangements of electron pairs around a central atom.
This shape typically arises with a central atom bonded to three other atoms and possessing two non-bonding electron pairs (lone pairs).
For the \( \mathrm{ClF}_{3} \) molecule:

• Chlorine (Cl) acts as the central atom, surrounded by three fluorines (F).
• The excess lone pairs on chlorine create repulsion, forcing the fluorine atoms into a T-shaped arrangement.

This unique shape can lead to varying bond lengths and partial charges, explaining why not all bonds are equal in \( \mathrm{ClF}_{3} \).
The T-shaped structure illustrates the influence of lone pairs in bond angles and molecular shape, making it an interesting study in chemical bonding.

Trigonal Planar Structure

The trigonal planar structure is a simple and symmetrical molecular geometry. Here, a central atom bonds with three surrounding atoms placed at the corners of an equilateral triangle.
This results in bond angles of exactly 120°.
For instance, in \( \mathrm{BF}_{3} \):

• Boron (B) is the central atom with fluorine (F) atoms evenly spaced around it.
• Due to symmetry, all B-F bonds are equal in length.

The trigonal planar structure exemplifies a perfect balance of bonds, leading to molecular stability and equal bond strength across the molecule.
These features make it a cornerstone in studying symmetrical molecules like \( \mathrm{BF}_3 \), showing how balance in geometry can simplify complex interactions.