The molecular geometry of PF3 is trigonal planar.
a. true
b. false
The Correct Answer and Explanation is:
Correct Answer: b. False
Explanation:
The molecular geometry of phosphorus trifluoride (PF₃) is trigonal pyramidal, not trigonal planar. This distinction arises from the electron geometry and the molecular shape determined by the VSEPR (Valence Shell Electron Pair Repulsion) theory.
1. Electron Configuration and Bonding:
Phosphorus (P) has five valence electrons, and each fluorine (F) atom contributes one electron for bonding. In PF₃:
- Phosphorus forms three single covalent bonds with fluorine atoms, using three of its valence electrons.
- The remaining two electrons form a lone pair on the phosphorus atom.
2. Electron Geometry:
The total number of regions of electron density (bonds and lone pairs) around the central phosphorus atom is four:
- Three bonding pairs (P-F bonds).
- One lone pair.
According to the VSEPR theory, four regions of electron density arrange themselves in a tetrahedral electron geometry to minimize repulsion.
3. Molecular Geometry:
While the electron geometry is tetrahedral, the molecular geometry is determined by the positions of the atoms, not the lone pairs. The lone pair on phosphorus pushes the three P-F bonds downward, creating a trigonal pyramidal shape.
4. Key Differences Between Trigonal Planar and Trigonal Pyramidal:
- Trigonal planar geometry involves three bonded atoms with no lone pairs on the central atom, resulting in bond angles of 120°. Example: BF₃.
- Trigonal pyramidal geometry involves three bonded atoms and one lone pair on the central atom, resulting in bond angles slightly less than 109.5° due to lone-pair repulsion. Example: PF₃.
5. Conclusion:
The molecular geometry of PF₃ is trigonal pyramidal, not trigonal planar, due to the lone pair on phosphorus, which distorts the structure. This distinction highlights the influence of lone pairs in shaping molecular geometry.