Here's why:

* Central Iodine: The central iodine atom forms two covalent bonds with the other two iodine atoms.

* Lone Pairs: The central iodine atom also has three lone pairs of electrons.

* VSEPR Theory: According to VSEPR theory, the electron pairs around the central atom arrange themselves to minimize repulsion. A linear arrangement of the three atoms allows for maximum separation of the electron pairs.

Therefore, the triiodide ion has a linear shape with the two terminal iodine atoms on opposite sides of the central iodine atom.