Problem 184 - A Complete Resource Book in Chemistry for JEE Main

Question

The number of lone pairs on \(\mathrm{Xe}\) in \(\mathrm{XeF}_{2}, \mathrm{XeF}_{4}\) and \(\mathrm{XeF}_{6}\) respectively are [2002] (a) \(3,2,1\) (b) \(2,4,6\) (c) \(1,2,3\) (d) \(6,4,2\)

Step-by-Step Solution

Answer

(c) 1, 2, 1

Step 1: Count Valence Electrons of Xenon

Xenon (Xe) belongs to group 18 in the periodic table and therefore has 8 valence electrons.

Step 2: Determine Total Electrons in Each Compound

For each xenon fluoride compound, calculate the total number of valence electrons by adding the valence electrons of xenon and those from the fluorine atoms. Each fluorine atom contributes 7 valence electrons.- In \(\mathrm{XeF}_2\): there are 8 (from Xe) + 2*7 (from 2 F) = 22 valence electrons.- In \(\mathrm{XeF}_4\): there are 8 (from Xe) + 4*7 (from 4 F) = 36 valence electrons.- In \(\mathrm{XeF}_6\): there are 8 (from Xe) + 6*7 (from 6 F) = 50 valence electrons.

Step 3: Determine Number of Bonds in Each Compound

Each Xenon-Fluorine bond uses 2 electrons. Calculate how many electrons are used in bonding:- In \(\mathrm{XeF}_2\): 2 bonds, total of 2*2 = 4 electrons.- In \(\mathrm{XeF}_4\): 4 bonds, total of 4*2 = 8 electrons.- In \(\mathrm{XeF}_6\): 6 bonds, total of 6*2 = 12 electrons.

Step 4: Calculate Lone Pairs on Xenon

Subtract the bonding electrons from the total valence electrons to determine the lone pairs:- In \(\mathrm{XeF}_2\): 22 total electrons - 4 bonding electrons = 18 non-bonding electrons. This gives 18/2 = 9 pairs, 8 of which go to the fluorines, leaving 1 lone pair on Xenon.- In \(\mathrm{XeF}_4\): 36 total electrons - 8 bonding electrons = 28 non-bonding electrons. This gives 28/2 = 14 pairs, 12 of which go to the fluorines, leaving 2 lone pairs on Xenon.- In \(\mathrm{XeF}_6\): 50 total electrons - 12 bonding electrons = 38 non-bonding electrons. This gives 38/2 = 19 pairs, 18 of which go to the fluorines, leaving 1 lone pair on Xenon.

Key Concepts

Understanding Lone Pairs

Lone pairs are electrons in a molecule that do not participate in bonding. They are crucial because their presence can affect the molecule's shape, reactivity, and sometimes stability.
When examining xenon compounds like \( \text{XeF}_2 \), \( \text{XeF}_4 \), and \( \text{XeF}_6 \), it's important to understand how these lone pairs manifest. Each lone pair consists of two electrons that are not shared with another atom.
In xenon fluoride compounds:

• In \( \text{XeF}_2 \), xenon has one lone pair since eight pairs go to fluorine.
• In \( \text{XeF}_4 \), xenon has two lone pairs as twelve pairs are allocated to fluorine.
• In \( \text{XeF}_6 \), with fluorine taking most, xenon is left with one lone pair.

Understanding how to count and calculate these can help predict how the compound will interact during chemical reactions.

Valence Electrons Explained

Valence electrons play a pivotal role in chemical bonding and properties of elements. They are the electrons found in the outermost shell of an atom, which are responsible for forming bonds with other atoms.
Xenon, despite being a noble gas with a stable configuration, participates in bonding under certain conditions by utilizing its valence electrons.
For xenon fluoride compounds:

• Xenon brings 8 valence electrons to the table because it's in group 18.
• Each fluorine atom contributes 7 more valence electrons, aiding the formation of the molecule's structure.

Valence electrons determine much about the atom's reactivity, enabling it to form multiple types of bonds depending on the number of available electrons.

Bonding in Molecules

Bonding in molecules occurs to achieve a more stable electron configuration. In the case of xenon compounds, the bonding involves the sharing of xenon's and fluorine's electrons.
Each bond between xenon and fluorine uses two electrons—one from xenon and one from fluorine. This sharing allows for the formation of stable molecular structures.

• In \( \text{XeF}_2 \), two fluorines bond to xenon using four total electrons.
• In \( \text{XeF}_4 \), four fluorines bond, using eight electrons in the process.
• For \( \text{XeF}_6 \), it's a higher count with six bonds using twelve electrons.

Understanding bonding is crucial as it not only stabilizes the atoms within the molecule but also defines the molecule's geometry and its chemical behavior. This is why counting and allocating electrons accurately is essential when assessing potential interactions of xenon compounds.