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 to minimize repulsion, leading to a specific electron domain geometry. The molecular geometry is then determined by the arrangement of only the atoms (bonding pairs).

Why (C) is correct:

1. Determine the central atom: Xenon (Xe) is the central atom in ${\mathrm{XeF}}_4$.

2. Calculate the number of valence electrons: Xenon is a noble gas, so it has 8 valence electrons. Each fluorine atom contributes 1 electron for bonding. There are 4 fluorine atoms.

3. Calculate the total number of electron pairs around the central atom:
Number of bonding pairs = 4 (for the four Xe-F bonds)
Number of lone pairs = (Valence electrons of Xe - electrons used in bonding) / 2
Number of lone pairs = (8 - 4) / 2 = 4 / 2 = 2

4. Determine the steric number: Steric number = Number of bonding pairs + Number of lone pairs = 4 + 2 = 6.

5. Determine the electron domain geometry: A steric number of 6 corresponds to an octahedral electron domain geometry.

6. Determine the molecular geometry: With 4 bonding pairs and 2 lone pairs in an octahedral arrangement, the two lone pairs occupy positions opposite to each other to minimize repulsion. This arrangement results in a square planar molecular geometry for the atoms.

Option Analysis:

• A) planar and tetrahedral: Incorrect. The electron domain geometry is octahedral, not tetrahedral.
• B) tetrahedral and planar: Incorrect. The electron domain geometry is octahedral, not tetrahedral, and the order is reversed.
• C) square planar and octahedral: Correct. As derived above, the molecular geometry is square planar, and the electron domain geometry is octahedral.
• D) pyramidal and tetrahedral: Incorrect. Neither geometry matches the VSEPR prediction for ${\mathrm{XeF}}_4$.

Correct Answer: (C)