Draw Lewis structures for the molecules IF5 and PF5 . How do the molecular shapes differ? In IF5 there is a lone pair of electrons on iodine where as in PF5 there are no lone pairs on phosphorus. As a result, the molecular shape of IF5 is square pyramidal and PF5 is trigonal bipyramidal. In PF5 there is a lone pair of electrons on phosphorus where as in IF5 there are no lone pairs on iodine. As a result, the molecular shape of PF5 is square pyramidal and IF5 is trigonal bipyramidal In IF5 there is a lone pair of electrons on iodine where as in PF5 there are no lone pairs on phosphorus. As a result, the molecular shape of IF5 is octahedral and PF5 is trigonal bipyramidal
The correct answer and explanation is:
Correct Answer:
In IF₅, there is a lone pair of electrons on iodine, whereas in PF₅, there are no lone pairs on phosphorus. As a result, the molecular shape of IF₅ is square pyramidal, and PF₅ is trigonal bipyramidal.
Explanation (≈300 words):
The Lewis structure of a molecule shows how valence electrons are arranged among the atoms. To determine molecular shape, we consider both bonding pairs and lone pairs of electrons using VSEPR (Valence Shell Electron Pair Repulsion) theory.
Phosphorus pentafluoride (PF₅):
Phosphorus (P) has five valence electrons and forms five single bonds with fluorine (F) atoms. Fluorine atoms each form one bond and hold three lone pairs. Since phosphorus uses all five valence electrons to bond with fluorine atoms, there are no lone pairs on the phosphorus atom in PF₅. This results in five bonding pairs, which arrange themselves to minimize repulsion in a trigonal bipyramidal geometry. In this structure, three fluorine atoms occupy the equatorial positions (in a plane 120° apart), and two occupy the axial positions (above and below the plane, 90° from the equatorial atoms).
Iodine pentafluoride (IF₅):
Iodine (I) has seven valence electrons and forms five single bonds with fluorine atoms. After bonding, iodine still has one lone pair. Therefore, IF₅ has six electron domains (five bonding pairs + one lone pair). According to VSEPR theory, six electron domains form an octahedral electron geometry. However, the lone pair slightly distorts the shape, resulting in a square pyramidal molecular geometry. Here, four fluorine atoms form the base of a square, and the fifth occupies the apex of the pyramid, while the lone pair occupies one position, repelling the bonded atoms and slightly compressing bond angles.
Summary:
- PF₅: No lone pairs on phosphorus → Trigonal bipyramidal shape
- IF₅: One lone pair on iodine → Square pyramidal shape
This difference in lone pairs leads to different molecular geometries despite both molecules having five bonded atoms.