Problem 147 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following molecules: (1) \(\mathrm{XeO}_{3}\) (2) \(\mathrm{XeOF}_{4}\) (3) \(\mathrm{XeF}_{6}\) Those having same number of lone pairs on \(\mathrm{Xe}\) are (a) (2) and (3) only (b) (1) and (2) only (c) \((1),(2)\) and (3) only (d) (1) and (3) only

Step-by-Step Solution

Answer

(c) All molecules, (1), (2), and (3) have the same number of lone pairs.

Step 1: Identify Valence Electrons for Xe

Xenon (Xe) is a noble gas with 8 valence electrons. We need to account for all valence electrons in each molecule by examining bonds and lone pairs.

Step 2: Calculate Lone Pairs for XeO3

In XeO_{3}, Xe makes 3 double bonds with oxygen, using up 6 of its valence electrons. This leaves 2 electrons or 1 lone pair on Xe.

Step 3: Calculate Lone Pairs for XeOF4

In XeOF_{4}, Xe forms 4 single bonds with fluorines and 1 double bond with oxygen, using 10 electrons. However, since the structure changes with bonding, Xe can expand its octet accommodating all bonds and keep 1 lone pair.

Step 4: Calculate Lone Pairs for XeF6

In XeF_{6}, Xe forms 6 single bonds with fluorines, accounting for 12 valence electron spaces. Xenon can expand its octet to accommodate extra electrons, leaving 1 lone pair remaining.

Step 5: Compare Lone Pairs

By analyzing the lone pairs, we find: XeO_{3} has 1 lone pair, XeOF_{4} has 1 lone pair, and XeF_{6} has 1 lone pair. All molecules contain similar number of lone pairs on Xe.

Key Concepts

Lone Pairs

Lone pairs are non-bonding pairs of electrons that reside in the valence shell of an atom. They are not shared with other atoms, meaning they do not participate in bonding. However, they significantly affect the molecule's geometry and reactivity. In the case of XeO₃, XeOF₄, and XeF₆, identifying lone pairs involves determining how many valence electrons xenon (Xe) has left after forming bonds with surrounding atoms.

Understanding the presence of lone pairs is crucial because:

• Lone pairs affect molecular geometry by repelling bonding pairs, altering the shape of the molecule.
• They contribute to the electron distribution around the atom, influencing properties like polarity and reactivity.

In this exercise, each xenon atom in XeO₃, XeOF₄, and XeF₆ has one lone pair after bonding with the respective atoms, illustrating a commonality among these molecules.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are essential for bonding. They determine how an atom can interact chemically with other atoms. Xenon, being a noble gas, has 8 valence electrons in its outer shell. These electrons are either shared with other atoms or remain as lone pairs when forming compounds.

In xenon compounds:

• XeO₃: The molecule uses 6 valence electrons for bonding with oxygen (in double bonds), leaving 2 electrons that form one lone pair.
• XeOF₄: Here, xenon utilizes valence electrons to form 4 single bonds with fluorine and 1 double bond with oxygen, expanding its octet but leaving one lone pair untouched.
• XeF₆: It uses valence electrons to form 6 single bonds with fluorine, accounting for 12 electron spaces, and due to its ability to expand the octet, retains one lone pair.

Understanding valence electrons helps in predicting and explaining the bonding and resultant number of lone pairs in these molecules.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It is influenced by the electron pairs around the central atom, including both bonding and lone pairs. The VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict the shapes of molecules by minimizing the repulsion between electron pairs.

For instance, in these xenon compounds:

• XeO₃: The presence of three double bonds and one lone pair leads to a trigonal pyramidal geometry due to the repulsion from the lone pair.
• XeOF₄: With four single bonds and a lone pair, the geometry can be described as square pyramidal, resulting from lone pair-bond pair repulsion.
• XeF₆: The octahedral arrangement is modified by the lone pair's presence, resulting in a distorted octahedral or square bipyramidal structure.

Recognizing the impact of lone pairs and bonding pairs on molecular geometry is crucial for understanding the spatial configuration and properties of these compounds.