Question

Which among the following species has unequal bond lengths?

• A. XeF$_4$
• B. SiF$_4$
• C. SF$_4$
• D. BF$_4^-$

Hint:

According to VSEPR theory, molecules with a steric number of 5 (trigonal bipyramidal electron geometry) often have unequal bond lengths (axial vs. equatorial), especially if there are lone pairs present, as in SF$_4$ (see-saw) or ClF$_3$ (T-shaped).

Answer 1

The Correct Option is C

We need to determine the molecular geometry of each species using VSEPR theory to identify which one has unequal bond lengths.
(A) XeF$_4$: Xenon (Group 18) has 8 valence electrons. It forms 4 bonds with F and has 2 lone pairs. The steric number is 6. The geometry is AX$_4$E$_2$, which corresponds to a square planar shape. In a square planar geometry, all four bonds are identical in length.
(B) SiF$_4$: Silicon (Group 14) has 4 valence electrons. It forms 4 bonds with F and has 0 lone pairs. The steric number is 4. The geometry is AX$_4$, which is tetrahedral. In a tetrahedral geometry, all four bonds are identical.
(C) SF$_4$: Sulfur (Group 16) has 6 valence electrons. It forms 4 bonds with F and has 1 lone pair. The steric number is 5. The geometry is AX$_4$E$_1$, which is a see-saw shape based on a trigonal bipyramidal arrangement. This geometry has two types of positions: two axial positions and two equatorial positions. The axial S-F bonds are longer than the equatorial S-F bonds due to greater repulsion from the lone pair in the equatorial plane. Therefore, SF$_4$ has unequal bond lengths.
(D) BF$_4^-$: Boron (Group 13) has 3 valence electrons, plus 1 from the negative charge, for a total of 4. It forms 4 bonds with F and has 0 lone pairs. The steric number is 4. The geometry is AX$_4$, which is tetrahedral. All four bonds are identical.
Thus, SF$_4$ is the species with unequal bond lengths.