Question

According to VSEPR theory, which of the following species has a see-saw shape?

• A. \(\text{SF}_4\)
• B. \(\text{XeF}_4\)
• C. \(\text{CCl}_4\)
• D. \(\text{BF}_4^-\)

Hint:

A see-saw shape is the classic signature of an \(sp^3d\) hybridized central atom (Steric Number 5) that possesses exactly 4 bond pairs and 1 lone pair (e.g., \(\text{SF}_4\), \(\text{TeCl}_4\)).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
We need to determine the molecular shape of each given species using the Valence Shell Electron Pair Repulsion (VSEPR) theory to find which one adopts a see-saw geometry.

Step 2: Key Formula or Approach:
First, calculate the steric number for the central atom:
\(\text{Steric Number (SN)} = \frac{1}{2} (\text{Valence } e^- \text{ on central atom} + \text{Number of monovalent atoms} - \text{Cation charge} + \text{Anion charge})\).
Based on the SN and the number of lone pairs, determine the geometry and shape.

Step 3: Detailed Explanation:
Let's evaluate each option:
-

(A) \(\text{SF}_4\): Sulfur (S) has 6 valence electrons. It forms 4 single bonds with F atoms.
\(\text{SN} = \frac{1}{2}(6 + 4) = 5\).
There are 4 bond pairs and \(5 - 4 = 1\) lone pair.
The electron geometry is trigonal bipyramidal. Placing the lone pair in an equatorial position to minimize repulsion results in a

see-saw shape.
-

(B) \(\text{XeF}_4\): Xenon (Xe) has 8 valence electrons. It forms 4 single bonds with F atoms.
\(\text{SN} = \frac{1}{2}(8 + 4) = 6\).
There are 4 bond pairs and \(6 - 4 = 2\) lone pairs.
The electron geometry is octahedral. The two lone pairs occupy opposite positions, resulting in a

square planar shape.
-

(C) \(\text{CCl}_4\): Carbon (C) has 4 valence electrons. It forms 4 single bonds with Cl atoms.
\(\text{SN} = \frac{1}{2}(4 + 4) = 4\).
There are 4 bond pairs and 0 lone pairs.
The shape is strictly

tetrahedral.
-

(D) \(\text{BF}_4^-\): Boron (B) has 3 valence electrons. There are 4 monovalent F atoms and a charge of \(-1\).
\(\text{SN} = \frac{1}{2}(3 + 4 + 1) = 4\).
There are 4 bond pairs and 0 lone pairs.
The shape is

tetrahedral.
Thus, only \(\text{SF}_4\) possesses a see-saw shape.

Step 4: Final Answer:
The correct choice is (A).