Which of the following molecules has a trigonal pyramidal shape?
a.COCl2
b. CH4
c. PC13
d. C13
e. BrF3
The Correct Answer and Explanation is :
The molecule with a trigonal pyramidal shape among the given options is:
c. PCl₃
Explanation (300 words)
To determine the molecular shape, we apply the VSEPR (Valence Shell Electron Pair Repulsion) theory. This theory predicts the 3D arrangement of atoms in a molecule by minimizing electron pair repulsion around the central atom.
Analysis of the Options:
- COCl₂: This molecule has a central carbon atom with a double bond to oxygen and single bonds to two chlorine atoms. The central atom has no lone pairs, resulting in a trigonal planar geometry.
- CH₄: Methane has a central carbon atom bonded to four hydrogen atoms. The central carbon has no lone pairs, leading to a tetrahedral geometry.
- PCl₃: Phosphorus trichloride has a central phosphorus atom bonded to three chlorine atoms. The phosphorus atom has one lone pair of electrons. This lone pair repels the bonded atoms, causing a trigonal pyramidal shape.
- Cl₃: This option does not correspond to a molecular structure; it’s likely an error. If this refers to a Cl₃⁻ ion (chlorine trimer), its geometry would depend on specific bonding, but it does not form a stable trigonal pyramidal structure.
- BrF₃: Bromine trifluoride has a central bromine atom with three bonded fluorine atoms and two lone pairs. These lone pairs occupy equatorial positions, resulting in a T-shaped molecular geometry, not trigonal pyramidal.
Why PCl₃ is Trigonal Pyramidal:
In PCl₃, the phosphorus atom is surrounded by 5 valence electrons. Three are used for bonding with chlorine, leaving one lone pair. The lone pair creates stronger repulsion compared to bond pairs, resulting in a trigonal pyramidal geometry. The bond angle is slightly less than 109.5° due to this repulsion. This shape is characteristic of molecules with three bonded atoms and one lone pair around the central atom.