According to VSEPR theory, what is the shape of the PH3 molecule best described as?
The Correct Answer and Explanation is :
According to VSEPR (Valence Shell Electron Pair Repulsion) theory, the shape of the PH₃ (phosphine) molecule is best described as trigonal pyramidal.
Explanation:
VSEPR theory predicts the shape of a molecule based on the repulsion between electron pairs around the central atom. In the case of PH₃, phosphorus (P) is the central atom, and it is bonded to three hydrogen (H) atoms. Here’s a breakdown of how we arrive at the shape:
- Electron Domain Count: Phosphorus has five valence electrons, and each hydrogen atom contributes one electron, making a total of eight electrons around phosphorus (5 from phosphorus and 3 from hydrogen). These electrons will form bonding pairs with hydrogen atoms and potentially lone pairs on the central phosphorus atom.
- Electron Pairs and Geometry:
- There are three bonding pairs (from the three P-H bonds).
- Phosphorus also has one lone pair of electrons.
- According to VSEPR theory, the three bonding pairs and one lone pair on phosphorus will repel each other to minimize electron-electron repulsion.
- Shape Determination:
- The ideal geometry for four electron pairs (3 bonding pairs + 1 lone pair) around a central atom is tetrahedral.
- However, lone pairs of electrons exert more repulsion than bonding pairs, so the presence of a lone pair pushes the bonding pairs closer together, creating a trigonal pyramidal shape rather than a perfect tetrahedral geometry.
- The bond angles in PH₃ are approximately 93.5°, which is less than the ideal tetrahedral angle of 109.5° because of the lone pair’s repulsive effect.
- Electron Pair Geometry vs. Molecular Geometry:
- The electron pair geometry (which includes both bonding and lone pairs) is tetrahedral.
- The molecular geometry (considering only the positions of atoms) is trigonal pyramidal.
In conclusion, the PH₃ molecule adopts a trigonal pyramidal shape due to the presence of a lone pair on the phosphorus atom that repels the bonding pairs, distorting the molecule from a tetrahedral configuration.