Tri-iodide ion has a linear symmetry with bond-angle of 180°. In the structure of ${\mathrm{I}}_3^{-}$ ion :

Geometry of the ${\mathrm{I}}_3$ molecule is linear. Three iodine atoms make up the compound, but because one of them has a negative charge, there are 3 lone pairs of electrons and 2 bond pairs. It will have a steric number of 5. The three lone couples will move into equatorial positions and repel one another.