Problem 190 - A Complete Resource Book in Chemistry for JEE Main

Question

Number of lone pairs of electrons on Xe atoms in \(\mathrm{XeF}_{2}, \mathrm{XeF}_{4}\) and \(\mathrm{XeF}_{6}\) molecules are respectively [2002] (a) \(2,3,1\) (b) \(3,2,1\) (c) \(3,2,0\) (d) \(4,3,2\)

Step-by-Step Solution

Answer

(b) 3, 2, 1

Step 1: Determine the total valence electrons for Xe

Xenon (Xe) belongs to Group 18 of the periodic table, which means it has 8 valence electrons. Fluorine (F) is in Group 17 and has 7 valence electrons. However, when forming compounds, only the valence electrons of the central atom, Xe, are dynamically involved in determining lone pairs.

Step 2: Analyze XeF2

XeF2 consists of 1 Xe atom and 2 F atoms. The central Xe shares 2 of its valence electrons with the F atoms. Hence, 4 electrons are used in bond formation out of the 8 valence electrons. The remaining 4 electrons on Xe form 2 lone pairs on the Xe atom because each lone pair requires 2 electrons.

Step 3: Analyze XeF4

XeF4 consists of 1 Xe atom and 4 F atoms. The Xe atom forms bonds with 4 F atoms using 8 electrons (2 for each bond). Subsequently, the 8 available valence electrons of Xe are split as 8 (2 for each of the 4 single bonds). To distribute the remaining electrons, all valence electrons are used in bonding, leaving 4 electrons individually as 2 lone pairs on Xe.

Step 4: Analyze XeF6

XeF6 consists of 1 Xe atom and 6 F atoms. Each F atom forms a single bond with Xe using 2 electrons per bond (total 12 electrons). However, only 8 of Xe's electrons are available, resulting in 12 (2 per bond) for the 6 bonds. Thus, the 8 electrons of the Xe create 6 shared bonds out of 12 needed, maintaining 1 lone pair on Xe and accounting for extra stability of electrons in bonds.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in bonding. For Xenon (Xe), which is part of Group 18 on the periodic table, there are 8 valence electrons. These electrons are important because they determine how Xe can form bonds with other elements such as fluorine (F). In molecules like XeF₂, XeF₄, and XeF₆, knowing the number of valence electrons helps predict how many electrons will participate in bond formation and how many will be left as lone pairs on the Xe atom.

• Xenon has 8 valence electrons.
• Fluorine has 7 valence electrons.
• Understanding valence electrons helps us figure out molecule formation.

For the central Xe atom, only its valence electrons are typically considered for bonding and the formation of lone pairs, making it a key factor in determining the structure of the resulting compounds.

Lewis Structures

Lewis structures are diagrams that visually represent the valence electrons around atoms in a molecule. These structures help us understand how atoms are connected in a molecule, what kinds of bonds are formed, and how lone pairs are organized.

• They show bonds as lines between atoms.
• Lone pairs are depicted as pairs of dots.
• Help visualize molecular geometry and electron arrangement.

For Xenon compounds like XeF₂, XeF₄, and XeF₆, Lewis structures can clarify where valence electrons are used, either in forming bonds with fluorine atoms or remaining as lone pairs on Xenon. This distribution dictates the shape and stability of the molecule.

XeF2

XeF₂, or xenon difluoride, consists of one central Xe atom and two F atoms. Using its 8 valence electrons, the Xe atom contributes 4 electrons to form two bonds, one with each F atom. This results in two pairs of electrons being used for bonding. The remaining 4 electrons of Xenon form 2 lone pairs, each consisting of 2 electrons.

• Structure: One Xe bonded to two F atoms.
• 4 electrons used in two bonds.
• 4 electrons left as 2 lone pairs.

This arrangement gives the XeF₂ molecule a linear shape, as the lone pairs take up space better aligned to repel each other and the bonds evenly, which is a key feature in determining molecular structure.

XeF4

XeF₄, or xenon tetrafluoride, includes one Xe atom bonded to four F atoms. The Xe atom uses its entire set of 8 valence electrons to create 4 bonds, using 2 electrons per bond. Each of these bonds connects an F atom to the central Xe. Consequently, no valence electrons are left unused, and as a result, there are 2 lone pairs on the Xe.

• Structure: One Xe bonded to four F atoms.
• 8 electrons fully used in four single bonds.
• Remaining 4 electrons create 2 lone pairs.

The presence of the 2 lone pairs in XeF₄ introduces a square planar shape, where the lone pairs sit opposite each other, minimizing repulsion between them and maintaining molecular stability.

XeF6

XeF₆, or xenon hexafluoride, involves a single Xe atom connected to six F atoms. The central Xe atom engages its full capacity of 8 valence electrons to bond with the F atoms. However, XeF₆ requires a total of 12 bonding electrons (2 electrons for each of the 6 bonds), resulting in a unique situation where Xenon involves 1 lone pair even after forming these bonds.

• Structure: One Xe bonded to six F atoms.
• Xenon's 8 valence electrons produce 6 bonds.
• 1 lone pair remains on Xe.

This arrangement results in a distorted octahedral shape. The single lone pair slightly affects the geometry, creating an imbalance that results in the shape being less symmetrical compared to perfect octahedral symmetry without lone pairs. Understanding this helps illustrate how complex electron arrangements can impact the spatial structure of molecules.