Problem 80 - A Complete Resource Book in Chemistry for JEE Main

Question

Total number of lone pair of electrons in \(\mathrm{XeOF}_{4}\) is (a) 0 (b) 1 (c) 2 (d) 3

Step-by-Step Solution

Answer

(b) 1

Step 1: Determine the Total Valence Electrons

First, determine the total number of valence electrons for the molecule \(\mathrm{XeOF}_4\). Xenon (Xe) is a noble gas and contributes 8 valence electrons. Oxygen (O) contributes 6 valence electrons, and each fluorine (F) provides 7 valence electrons. There are four fluorine atoms, so they contribute 28 electrons. Thus, the total number of valence electrons is \(8 + 6 + 28 = 42\) electrons.

Step 2: Construct the Lewis Structure

Using the 42 valence electrons, construct the Lewis structure for \(\mathrm{XeOF}_4\). Place xenon in the center, bonded to one oxygen and four fluorine atoms. Each bond accounts for 2 electrons. Therefore, there are \(5 \times 2 = 10\) electrons used in forming bonds.

Step 3: Calculate Remaining Electrons

Subtract the number of electrons used in bonds from the total number of valence electrons to find the remaining electrons: \(42 - 10 = 32\) electrons.

Step 4: Distribute Remaining Electrons as Lone Pairs

The remaining 32 electrons should be placed around the outer atoms to satisfy their octets. Oxygen and each fluorine have 6 electrons as lone pairs in addition to the bonding pair, which satisfies the octet rule. This uses \(4 \text{ F atoms} \times 6 \text{ electrons} + \text{O atom} \times 6 \text{ electrons} = 30\) electrons in lone pairs.

Step 5: Identify Lone Pairs on Xenon

After placing electrons around the peripheral atoms, 2 electrons remain: - Assign these 2 electrons as a lone pair on xenon. Since all electrons have been distributed, xenon has one lone pair.

Key Concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. They play a crucial role in the chemical properties and bonding behavior of an element. For example, in the molecule \( \mathrm{XeOF}_4 \), determining the total number of valence electrons is the first step to understand how the atoms bond:

• Xenon (Xe), being a noble gas, has 8 valence electrons.
• Oxygen (O) has 6 valence electrons.
• Each of the four fluorine (F) atoms has 7 valence electrons, summing up to 28 valence electrons for all four.

By adding these numbers together, you determine that \( \mathrm{XeOF}_4 \) has a total of 42 valence electrons. This count is foundational for constructing the molecule's structure because it indicates how electrons can be shared or used as lone pairs.

Lewis Structure

The Lewis structure is a visual representation of the bonding between atoms in a molecule and the lone pairs of electrons that may exist. To construct the Lewis structure for \( \mathrm{XeOF}_4 \), you follow a few logical steps:

• Place Xenon (Xe) as the central atom.
• Connect it via single bonds to the Oxygen (O) and four Fluorine (F) atoms.

Every bond represents a pair of electrons shared between the involved atoms. With \( \mathrm{XeOF}_4 \), forming these bonds consumes 10 electrons (5 bonds x 2 electrons per bond). After the bonds are established, you move forward to distribute the remaining electrons, keeping the octet rule in mind.

Bonding Pairs

Bonding pairs are the pairs of electrons shared between two atoms to form a covalent bond. In \( \mathrm{XeOF}_4 \), the central Xenon atom forms five bonds:

• One bond with the Oxygen atom
• Four bonds with Fluorine atoms

Each of these bonds is a bonding pair, accounting for 10 electrons from the original pool of 42 valence electrons. These shared electrons are crucial in holding the molecule together, providing stability and forming a basis for the structure's geometric shape. Ensuring that these bonding pairs are correctly established is key to later distributing any remaining valence electrons as lone pairs.

Octet Rule

The octet rule is a chemical rule of thumb stating that atoms tend to bond in a manner that each atom has eight electrons in its valence shell, reflecting a noble gas configuration. While not absolute, this rule guides the placement of electrons, especially lone pairs, around atoms after bonding pairs are constructed. For \( \mathrm{XeOF}_4 \):

• Each Fluorine needs an additional 6 electrons to satisfy its octet, having one bond already.
• Oxygen also needs 6 extra electrons because of its single bond with Xenon.

After ensuring that oxygen and fluorine follow the octet rule and consuming 30 electrons, the remaining electrons become lone pairs on the central Xenon atom, demonstrating a unique case where Xenon can have more than 8 electrons.