Question

In which of the following molecules, all bond lengths are not equal?

• A. SF$_6$
• B. PCl$_5$
• C. BCl$_3$
• D. CCl$_4$

Hint:

In molecules with different bond positions (like trigonal bipyramidal), axial bonds are usually longer than equatorial bonds.

Answer 1

The Correct Option is B

Step 1: Understanding the Molecular Geometry
PCl$_5$ has a trigonal bipyramidal structure with two types of bond positions: axial and equatorial.
The three chlorine atoms in the equatorial plane are bonded at \( 120^\circ \), while the two axial chlorine atoms are at \( 90^\circ \) with respect to the equatorial bonds.
Step 2: Bond Length Difference
Due to repulsion effects, axial bonds are longer than equatorial bonds.
The axial bonds experience greater repulsion from equatorial bonds, causing them to be longer.
Final Answer: PCl$_5$ has unequal bond lengths due to its trigonal bipyramidal shape.

Answer 2

In the molecule \( \text{PCl}_5 \), all bond lengths are not equal:

Phosphorus pentachloride (\( \text{PCl}_5 \)) has a trigonal bipyramidal structure, where the phosphorus atom is at the center bonded to five chlorine atoms.

Due to this geometry, the bond lengths differ based on the position of the chlorine atoms:
- The three chlorine atoms in the equatorial plane have bond lengths that are shorter.
- The two chlorine atoms in the axial positions have longer bond lengths because of greater repulsion and spatial arrangement.

Therefore, in \( \text{PCl}_5 \), axial P–Cl bonds are longer than equatorial P–Cl bonds, making the bond lengths unequal.

In contrast, molecules like \( \text{BF}_3 \) or \( \text{CH}_4 \) have all bonds equal due to their symmetric geometries.