Question

According to the VSEPR theory, what are the most stable shapes of \(\text{XeF}_4\) and \(\text{SF}_4\), respectively?

• A. Square planar and see-saw
• B. Both see-saw
• C. See-saw and square planar
• D. Both square planar

Hint:

For a quick recall of steric configurations:
- \(\text{AB}_4\text{E}_2\) systems (like \(\text{XeF}_4\)) are always square planar.
- \(\text{AB}_4\text{E}\) systems (like \(\text{SF}_4\)) are always see-saw.
This simple classification covers most of the shape questions in exams.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the molecular shapes of Xenon tetrafluoride (\(\text{XeF}_4\)) and Sulfur tetrafluoride (\(\text{SF}_4\)) based on Valence Shell Electron Pair Repulsion (VSEPR) theory.

Step 2: Key Formula or Approach:
According to VSEPR theory, molecular shape is determined by the total steric number (SN):
\(\text{Steric Number (SN)} = \) \[\text{Number of single bonds (bond pairs)} + \text{Number of lone pairs on the central atom} \]
The spatial arrangement minimizes repulsive interactions between electron pairs in the order:
\[ \text{lone pair - lone pair} > \text{lone pair - bond pair} > \text{bond pair - bond pair} \]

Step 3: Detailed Explanation:
Let's analyze both molecules individually:
1. Xenon tetrafluoride (\(\text{XeF}_4\)):
- Central atom: Xenon (\(\text{Xe}\)), which has 8 valence electrons.
- Number of fluorine atoms attached = 4 (4 bond pairs).
- Remaining valence electrons = \(8 - 4 = 4\) electrons, which form 2 lone pairs.
- Steric number = \(4\text{ bond pairs} + 2\text{ lone pairs} = 6\).
- Electronic geometry: Octahedral.
- To minimize lone pair-lone pair repulsion, the two lone pairs occupy the axial positions (opposite to each other).
- This leaves the four fluorine atoms in the equatorial plane, resulting in a stable square planar shape.
2. Sulfur tetrafluoride (\(\text{SF}_4\)):
- Central atom: Sulfur (\(\text{S}\)), which has 6 valence electrons.
- Number of fluorine atoms attached = 4 (4 bond pairs).
- Remaining valence electrons = \(6 - 4 = 2\) electrons, which form 1 lone pair.
- Steric number = \(4\text{ bond pairs} + 1\text{ lone pair} = 5\).
- Electronic geometry: Trigonal bipyramidal.
- To minimize repulsion, the lone pair occupies an equatorial position (where it experiences fewer \(90^\circ\) interactions).
- This arrangement results in a distorted trigonal bipyramid, commonly known as a see-saw shape.

Step 4: Final Answer:
The shapes of \(\text{XeF}_4\) and \(\text{SF}_4\) are square planar and see-saw, respectively, which corresponds to option (A).