The electron group geometry of I3 is trigonal bipyramidal; linear and its molecular geometry is trigonal bipyramidal; seesaw.
The Correct Answer and Explanation is:
To clarify the electron group geometry and molecular geometry of I3−I_3^-I3− (triiodide ion), let’s go step by step.
Electron Group Geometry of I3−I_3^-I3−
The electron group geometry refers to the spatial arrangement of all electron groups (bonding and lone pairs) around the central atom. For I3−I_3^-I3−, iodine is the central atom, and there are three iodine atoms attached to it.
- Total number of valence electrons:
- Iodine (I) is in Group 17, so it has 7 valence electrons.
- The I3−I_3^-I3− ion carries a negative charge, meaning there is an extra electron, bringing the total to 7×3+1=227 \times 3 + 1 = 227×3+1=22 valence electrons.
- Electron group arrangement:
The iodine atom has 3 bonding pairs (one for each iodine atom attached) and 2 lone pairs. These 5 electron groups (3 bonding pairs + 2 lone pairs) around iodine give the molecule a trigonal bipyramidal electron group geometry.
Molecular Geometry of I3−I_3^-I3−
While the electron group geometry refers to the arrangement of all electron groups (bonding and lone pairs), the molecular geometry describes only the arrangement of the atoms (ignoring the lone pairs).
- The two lone pairs occupy the equatorial positions of the trigonal bipyramidal geometry because these positions are more spatially favorable, minimizing repulsion.
- The three iodine atoms occupy the two axial positions and one equatorial position.
This gives I3−I_3^-I3− a linear molecular geometry, with a bond angle of 180° between the iodine atoms.
Summary
- Electron group geometry: Trigonal bipyramidal (due to 5 electron groups).
- Molecular geometry: Linear (due to the positioning of 3 iodine atoms and 2 lone pairs).
Thus, the structure of I3−I_3^-I3− is best described as linear for the molecular geometry, even though its electron group geometry is trigonal bipyramidal.
