The electron geometry of I3 (triiodide ion) is trigonal bipyramidal.
To understand why, we first need to look at the molecular structure of the ion. I3 consists of three iodine atoms. In the triiodide ion, there is one central iodine atom surrounded by two other iodine atoms. The central iodine atom has three lone pairs of electrons and forms two bonds with the other iodine atoms. This arrangement of electron pairs and bonded atoms affects the molecular geometry.
The trigonal bipyramidal geometry arises from the repulsion between the lone pairs of electrons and the bonding electrons. The three lone pairs occupy equatorial positions in this geometry, while the bonded iodine atoms occupy axial positions.
When considering electron pairs (both bonding and lone pairs), the arrangement is thought of in terms of minimizing repulsion through optimal spacing. In the case of I3, the best arrangement is a trigonal bipyramidal shape because it allows the lone pairs to be as far from each other as possible, thereby reducing electron pair repulsion.
So, in summary, the electron geometry of I3 is trigonal bipyramidal, primarily due to the arrangement of electron pairs around the central iodine atom.