If the compound phosphorus pentaiodide (PI5) has no lone pairs, what shape would it take?
The correct answer and explanation is:
Answer:
If phosphorus pentaiodide (PI₅) has no lone pairs on the central phosphorus atom, its molecular geometry would be trigonal bipyramidal.
Explanation:
To determine the shape of a molecule like phosphorus pentaiodide (PI₅), we start by looking at the central atom’s bonding and electron arrangement.
- Valence Electrons and Bonding:
Phosphorus (P) is in group 15 of the periodic table and has 5 valence electrons. In PI₅, phosphorus forms five single bonds with five iodine atoms. Each iodine atom brings one electron to form a bond with phosphorus, so the central phosphorus is bonded to five substituents. - Electron Domain Geometry:
Since phosphorus forms five bonds and we are told there are no lone pairs on the phosphorus, the total electron domains around the central atom are five bonding pairs and zero lone pairs. According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, five electron pairs arrange themselves as far apart as possible in a trigonal bipyramidal arrangement. - Trigonal Bipyramidal Shape:
- Geometry of Electron Domains: The five regions of electron density form a trigonal bipyramid, with three atoms arranged in an equatorial plane (120° apart) and two atoms positioned axially (180° apart from each other and 90° from the equatorial atoms).
- Molecular Shape: Since there are no lone pairs, the molecular shape is exactly the same as the electron domain geometry: trigonal bipyramidal.
- Why Not Octahedral or Other Shapes?
The octahedral shape corresponds to six regions of electron density. PI₅ only has five bonds, so it cannot be octahedral. If lone pairs were present, the shape would distort, but since none exist, the pure trigonal bipyramidal shape remains. - Realistic Considerations:
While PI₅ is a hypothetical or rarely encountered compound (because iodine atoms are large, making five bonds on phosphorus sterically crowded), the theoretical shape given the bonding and electron count is trigonal bipyramidal.
Summary:
- Phosphorus in PI₅ has 5 bonding pairs and 0 lone pairs.
- Electron domains: 5 → trigonal bipyramidal arrangement.
- No lone pairs means no distortion → molecular shape = trigonal bipyramidal.
- The shape has three iodine atoms in the equatorial plane and two in axial positions.
This geometry minimizes electron pair repulsions and best fits the VSEPR model for five bonded atoms and no lone pairs.