The Question is from NEET guide , and it was created by the author THIRUPATHI REDDY

Question

The $PCl_5$ molecule exhibits a trigonal bipyramidal geometry. Which of the following BEST explains the difference in bond lengths between axial and equatorial bonds in $PCl_5$?

THIRUPATHI REDDY · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: VSEPR theory and its application to molecular geometry and bond lengths.
 **Related Topics**: Hybridization, bond angles, electron repulsion, molecular geometry.
 **Prerequisites**: Understanding of valence shell electron pair repulsion (VSEPR) theory,  $sp^3d$ hybridization, and the concept of bond angles.
## SOLUTION PATHWAY
**Given Information**:
  $PCl_5$ molecule has a trigonal bipyramidal geometry.
  There is a difference in bond lengths between axial and equatorial P-Cl bonds.
**Method & Steps**:
 1. Analyze the trigonal bipyramidal geometry of $PCl_5$.
  Reasoning: In $PCl_5$, the phosphorus atom is surrounded by five chlorine atoms.  The geometry is trigonal bipyramidal, with three equatorial chlorine atoms and two axial chlorine atoms.
  Formula/Rule: VSEPR theory predicts that electron pairs (bonding and lone pairs) arrange themselves to minimize repulsion.
 2. Consider the repulsions between electron pairs.
  Working: Axial bond pairs experience greater repulsion because they are closer to the three equatorial bond pairs at 90° angles, than the equatorial bond pairs which only have two 90° interactions and one 120° interaction.  This increased repulsion leads to longer axial bonds compared to the equatorial bonds.
  Key Point: The 90° interactions are the dominant factor in determining the bond length difference.
 3. Evaluate the given options.
  Result: Option C correctly identifies the greater repulsion experienced by axial bond pairs due to 90° interactions with equatorial bond pairs as the best explanation for the difference in bond lengths.
  Verification: Other options are incorrect because the hybridization is the same for all Cl atoms ($sp^3d$), electron sharing is relatively equal, and while d-orbitals are involved in hybridization, they don't preferentially influence axial bonds over equatorial bonds in this way. 
 ## OPTION ANALYSIS
A :x: Difference in hybridization of axial and equatorial chlorine atoms is incorrect because all chlorine atoms have the same hybridization. 
B :x: Unequal sharing of electrons between phosphorus and axial vs. equatorial chlorine atoms is incorrect, as the electronegativity difference is the same for all P-Cl bonds.
C ✓: Greater repulsion experienced by axial bond pairs due to 90° interactions with equatorial bond pairs is the correct explanation.
D :x: The presence of d-orbitals in phosphorus preferentially influences axial bond lengths is incorrect; while d-orbitals are involved in hybridization, they don't cause the length difference.
 ## LEARNING AIDS
 **Common Mistakes**:
  1.Confusing hybridization with bond lengths: Hybridization affects bond angles, but not necessarily bond lengths directly.
  How to avoid: Focus on electron pair repulsions as the primary determinant of bond lengths in VSEPR theory.
  2.Ignoring the role of 90° interactions: Failing to recognize the significant difference in repulsions between 90° and 120° interactions.
  How to avoid: Carefully draw the 3D structure and analyze the angles between electron pairs.
 **Quick Tips**:
  1.Visualize the 3D structure: Drawing a 3D representation of the molecule helps to understand the spatial arrangement of atoms and electron pairs.
  2.Remember the key angles:  Axial-equatorial interactions are at 90°, and equatorial-equatorial interactions are at 120°.

THIRUPATHI REDDY · Answer

Difference in hybridization of axial and equatorial chlorine atoms.

THIRUPATHI REDDY · Answer

Unequal sharing of electrons between phosphorus and axial vs. equatorial chlorine atoms.

THIRUPATHI REDDY · Answer

The correct Option is "Greater repulsion experienced by axial bond pairs due to 90° interactions with equatorial bond pairs."

THIRUPATHI REDDY · Answer

The presence of d-orbitals in phosphorus preferentially influences axial bond lengths.

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