Explain the following :
Iodine forms `I_3^-` but `F_2^` does not form `F_3^-` ion. Why?

Explanation

`I_3^-` ion can be represented as follows:

In this ion, the central I-atom shows a covalency of two by expanding its octet. This is due to the availability of d-orbitals in the valence shell. On the other hand, fluorine, due to the non-availability of d-orbitals in the valence shell cannot expand its octet and hence cannot form `F_3^-` ion.