Understanding Molecular Geometry
Molecular geometry refers to the three-dimensional arrangement of atoms in a molecule. The shape of a molecule is determined by the number of bonding pairs and lone pairs of electrons around the central atom, which influences the overall geometry according to VSEPR (Valence Shell Electron Pair Repulsion) theory.
Pyramidal Shape of PCl3
1. Central Atom: In PCl3 (Phosphorus Trichloride), phosphorus (P) is the central atom.
2. Electron Pair Count: Phosphorus has five valence electrons. In PCl3:
- It forms three single bonds with three chlorine (Cl) atoms.
- This leaves one lone pair of electrons on the phosphorus atom.
3. Geometry: Due to the presence of one lone pair and three bonding pairs:
- The molecule adopts a trigonal pyramidal shape.
- The lone pair repels the bonding pairs, causing them to be pushed down, creating a pyramid-like structure.
4. Comparison with Other Options:
- SO3: Has a trigonal planar shape due to no lone pairs on sulfur.
- CO32-: Has a trigonal planar shape due to resonance and no lone pairs on carbon.
- NO3-: Also exhibits a trigonal planar shape, with no lone pairs on nitrogen.
Conclusion
Thus, the only molecule among the given options that exhibits a pyramidal shape is PCl3, making option 'A' the correct answer. This unique shape arises from the combination of bonding pairs and the lone pair on the central atom, following the principles of VSEPR theory.