Concept: VSEPR Theory

VSEPR (Valence Shell Electron Pair Repulsion) theory is used to predict the geometry of molecules based on the repulsion between electron pairs in the valence shell of the central atom. The electron pairs (both bonding and lone pairs) arrange themselves as far apart as possible to minimize repulsion.

Why (B) Distorted octahedral is correct:

1. Central Atom: Xenon (Xe) is the central atom.

2. Valence Electrons of Xe: Xenon is a noble gas, so it has 8 valence electrons.

3. Bonding Pairs: There are 6 fluorine (F) atoms bonded to Xe, forming 6 bonding pairs.

4. Lone Pairs: The number of lone pairs = (Valence electrons of central atom - Number of bonding electrons) / 2 = (8 - 6) / 2 = 1 lone pair.

5. Steric Number: The steric number (total number of electron pairs) = Number of bonding pairs + Number of lone pairs = 6 + 1 = 7.

6. Electron Geometry: A steric number of 7 corresponds to a pentagonal bipyramidal electron geometry.

7. Molecular Geometry: With 6 bonding pairs and 1 lone pair (AX₆E₁ type), the lone pair occupies an equatorial position in the pentagonal bipyramidal arrangement to minimize repulsion. This lone pair distorts the ideal octahedral geometry, leading to a distorted octahedral (or capped octahedral) molecular geometry.

Option Analysis:

• A) Octahedral: This would be the geometry if there were 6 bonding pairs and 0 lone pairs (AX₆ type), like in SF₆. However, XeF₆ has a lone pair.
• C) Planar: This geometry is associated with molecules having a steric number of 3 (e.g., BF₃) or certain square planar complexes (AX₄E₂). XeF₆ has a steric number of 7.
• D) Tetrahedral: This geometry is associated with a steric number of 4 (e.g., CH₄). XeF₆ has a steric number of 7.

Correct Answer: (B)