The geometry of `I_(3)^(-)` is

Explanation

The central I atom has the ground state electronic configuration as `5s^(2)5p^(5)`. It involves `sp^(3)d` hybridization involving one `5s`, three 5p and one vacant 5d orbital. The half filled `sp^(3)d` hybrid orbital forms a covalent bond with one I atom and an empty `sp^(3)d` hybrid orbital accepts an electron pair from `I^(-)` ion to form a coordinate bond. Thus, the geometry of the molecule is trigonal bipyramidal in which three positions are occupied by lone pairs.

Because of the equivalent positions of lone pairs the resulting `I_(3)^(-)` ion is linear.