MCQ Detailed View

The triiodide ion (I₃⁻) is a polyatomic ion consisting of three iodine atoms. Its molecular shape can be predicted using VSEPR (Valence Shell Electron Pair Repulsion) theory, which states that electron pairs around the central atom arrange themselves to minimize repulsion.

In I₃⁻, the central iodine atom is bonded to two other iodine atoms and also has three lone pairs of electrons. The electron pairs adopt a trigonal bipyramidal arrangement to minimize repulsion, but the lone pairs occupy the equatorial positions, leaving the two bonding pairs along the linear axis. As a result, the observed molecular shape of I₃⁻ is linear, with a bond angle of approximately 180°.

Other geometries like trigonal bipyramidal or square bipyramidal do not describe the actual shape of I₃⁻, as they ignore the influence of lone pairs on the central atom. Linear geometry in I₃⁻ is an example of how electron lone pairs influence molecular shape while maintaining symmetry.

The linear shape of I₃⁻ is significant in understanding halogen chemistry, polyhalide formation, and the bonding properties of polyatomic ions. It demonstrates how central atoms with expanded octets or multiple lone pairs can form molecules with unexpected geometries.

Understanding the molecular geometry of ions like I₃⁻ is essential in physical chemistry and helps predict reactivity, polarity, and interactions with other molecules or ions. The linear arrangement also explains why I₃⁻ is symmetric and nonpolar, despite containing polar I–I bonds.