Q.15
The shape of PCl5 is
Options:
(A) tetrahedral
(B) square planar
(C) trigonal bipyramidal
(D) square pyramidal
PCl5 (phosphorus pentachloride) molecular geometry questions test VSEPR theory and hybridization concepts fundamental to inorganic chemistry and materials science. With 5 chlorine atoms bonded to central phosphorus and no lone pairs, PCl5 adopts a distinctive 3D structure.
The correct answer is (C) trigonal bipyramidal, featuring sp³d hybridization, 3 equatorial Cl at 120° and 2 axial Cl at 90° to the plane.
Why (C) Trigonal Bipyramidal is Correct
Phosphorus (5 valence e⁻) forms 5 P-Cl bonds using sp³d hybridization (one s, three p, one d orbital). Steric number 5 (5 bonds, 0 lone pairs) dictates trigonal bipyramidal geometry per VSEPR theory—three equatorial positions (120° bond angles) and two axial positions (90° to equatorial).
Explanation of All Options
Each geometry corresponds to different electron pair arrangements:
-
(A) Tetrahedral
Wrong. Tetrahedral (sp³, steric #4) fits CH₄ or PCl₃ (3 bonds + 1 lone pair). PCl5 has 5 bonds, requiring extra orbital space. -
(B) Square planar
Incorrect. dsp² hybridization (steric #4 + 2 lone pairs), like XeF₄ or PtCl₄²⁻. PCl5 lacks square symmetry and d orbital pairing. -
(C) Trigonal bipyramidal
Correct. sp³d hybridization accommodates 5 bonds perfectly: 3 equatorial (120°), 2 axial (90°). Bond lengths differ (axial longer). -
(D) Square pyramidal
No. Would require octahedral electron geometry (6 pairs) with 1 lone pair, like BrF₅. PCl5 has only 5 bonding pairs.
Quick Geometry Comparison Table
| Option | Geometry | Steric # | Hybridization | Example Molecules | Fits PCl5? |
|---|---|---|---|---|---|
| A | Tetrahedral | 4 | sp³ | CH₄, SiF₄ | No |
| B | Square planar | 6 (4+2) | dsp² | XeF₄, PtCl₄²⁻ | No |
| C | Trigonal bipyramidal | 5 | sp³d | PCl₅, SbCl₅ | Yes |
| D | Square pyramidal | 6 (5+1) | sp³d² | BrF₅, IF₅ | No |
Biotech connection: PCl5 used in phosphorylation reactions for oligonucleotide synthesis. Visualize: axial bonds weaker, more reactive than equatorial in trigonal bipyramidal structure.
