This partial Lewis structure of the triiodide ion, I3-, allows us to focus on the central iodine atom which has an AB2E3 arrangement of electrons. [2] What type of hybridization is found on the central iodine atom in I3
The Correct Answer and Explanation is:
In the case of the triiodide ion, I3-, the central iodine atom has an AB2E3 electron arrangement. This notation tells us that iodine is surrounded by two bonding pairs (B) and three lone pairs (E) of electrons. Given this, the electron geometry around the iodine atom is trigonal bipyramidal.
Now, to determine the hybridization of the central iodine, we look at the number of electron groups around the atom. Each bonding pair and each lone pair counts as one electron group, regardless of whether the electrons are shared in bonds or are unbonded lone pairs.
Since iodine has two bonding pairs and three lone pairs, this results in five electron groups around the iodine atom. When there are five electron groups, the hybridization scheme required to form the appropriate bonding and lone pair arrangement is sp3d hybridization.
In sp3d hybridization, one s orbital, three p orbitals, and one d orbital mix together to form five hybrid orbitals that are directed in a trigonal bipyramidal arrangement. The bonding pairs of electrons occupy two of these orbitals, while the three lone pairs occupy the remaining orbitals, occupying equatorial positions in the trigonal bipyramidal geometry.
Thus, the central iodine atom in I3- has sp3d hybridization. This hybridization allows the iodine to accommodate both the bonding and lone pair electrons and arrange them according to the electron geometry of the triiodide ion. This structure also explains the linear shape of the I3- ion, as the lone pairs occupy positions that push the iodine atoms on either side into a straight line.
This type of hybridization is essential in understanding the geometry of molecules with five electron groups, which is common in larger molecules involving heavier elements like iodine.
