What is the electron pair geometry for a phosphine molecule, PH3? A) tetrahedral B) bent C) linear D) trigonal pyramidal E) none of the above
The Correct Answer and Explanation is :
The correct answer is D) trigonal pyramidal.
Explanation:
The electron pair geometry of a molecule is determined by the arrangement of electron pairs (both bonding and lone pairs) around the central atom. In the case of phosphine (PH₃), phosphorus (P) is the central atom, and it is surrounded by three hydrogen (H) atoms and one lone pair of electrons.
Step-by-step reasoning:
- Valence Electrons: Phosphorus belongs to group 15 of the periodic table, so it has five valence electrons. Each hydrogen atom contributes one electron, and phosphorus shares these electrons with hydrogen to form three P–H bonds.
- VSEPR Theory: According to the Valence Shell Electron Pair Repulsion (VSEPR) Theory, the geometry around the central atom is determined by minimizing the repulsion between electron pairs. In PH₃, there are four regions of electron density around phosphorus: three bonding pairs (P-H bonds) and one lone pair.
- Electron Pair Geometry: Ideally, with four regions of electron density, the electron pair geometry would be tetrahedral. However, since one of these regions is a lone pair, which exerts more repulsion than bonding pairs, the shape of the molecule is slightly distorted.
- Molecular Shape: The lone pair occupies more space than the bonding pairs, pushing the three P–H bonds closer together. This leads to a trigonal pyramidal molecular shape, where the phosphorus atom is at the apex and the three hydrogen atoms form the base of the pyramid. The bond angle is slightly less than the ideal tetrahedral angle (109.5°), typically around 93.5° in PH₃.
Therefore, the electron pair geometry remains tetrahedral, but the molecular geometry, which considers the positions of atoms only, is trigonal pyramidal.