The Correct Answer and Explanation is:
For question 34, the triiodide ion (I₃⁻) is linear because the central iodine atom has three lone pairs that effectively balance out the repulsions from surrounding atoms, allowing a straight arrangement. However, the triiodide cation (I₃⁺) lacks a lone pair on the central iodine, leading to an uneven distribution of electron density that forces the molecule into a bent shape.
For question 35, diborane (B₂H₆) exhibits D₂h symmetry because it has bridging hydrogen atoms that create a symmetrical arrangement around the boron centers. On the other hand, iodine trichloride (I₂Cl₆) adopts a planar configuration due to electron repulsions and steric effects that favor a flat structure.
Explanation
The shape of a molecule or ion is determined by its electronic configuration and the principles of valence shell electron pair repulsion (VSEPR) theory. In the case of I₃⁻, the central iodine atom has three lone pairs of electrons that symmetrically arrange themselves around it. Since lone pairs repel each other strongly, the molecule adopts a linear shape to minimize repulsions. Conversely, I₃⁺ has fewer lone pairs, and the bonding electrons do not distribute evenly enough to maintain linearity. This causes a bend in its shape, as lone-pair interactions no longer enforce strict linear geometry.
For B₂H₆, D₂h symmetry arises because the bridging hydrogen atoms are positioned in a staggered fashion, creating a balanced geometric framework. The boron atoms in diborane share electrons through three-center two-electron bonds, leading to a symmetrical, nonplanar configuration. Meanwhile, in I₂Cl₆, the large iodine atoms are surrounded by chlorine atoms in a manner that minimizes electron repulsions and steric hindrance, favoring a planar shape.
The structural differences between these compounds highlight how electron arrangement and molecular interactions dictate spatial orientation and stability.
