Iodine forms `I_(3)^(-)` but fluorine does not form `F_(3)^(-)` -why?

Explanation

Due to availability of vacant d -orbitals in iodine, `I_(2)` can form `I_(3)^(-)` ion by accepting electrons from `I^(-)` . However the formation of `F_(3)^(-)` by the combination of `F_(2)` with `F^(-)` is not possible because fluorine has no vacant d -orbital.