Halogens or interhalogens combine with halide ions to form polyhalide ions. The most common example of polyhalide ion formation is furnished by the increase in solubility of iodine in water in the presence of which is due to the formation of tri iodide ion, I^–₃

I^– + I₂ ———> I^–₃

Many other examples of polyhalides ions are

(i) Cl₃^–, Br^–₃, ICI^˜₂, IBr^–₂ including I₃^– . In these ions, one of the halogen atoms (in case of similar atoms) or halogen atom larger in size undergoes sp³d -hybridization giving a linear shape with three lone pairs at equatorial positions.

(ii) Cl₃⁺, Br₃⁺, I₃⁺, ICI₃⁺, IBr₂⁺ Here we find central atom sp³ hybridized giving a bent shape with two lone pairs of electrons on the central atom.

(iii) ICI₄^–, BrF₄^–, I₃^– . Here central atom involves sp³d² hybridization giving square planar shape with two lone pairs of electrons on axial positions.

(iv) ICI₄^–, BrF₄^–, I₅^–. In these ions central atom involves sp³d hybridization giving a distorted tetrahedral structure with one lone pair of electrons on equatorial position.

(v) I₇^–, IF₆⁺. The central I atom undergoes sp³d³ hybridization giving a distorted octahedral structure with one lone pair of electrons.

(vi) I₇⁺, Here central I atom involves sp³d² hybridization giving an octahedral structure.

Fluorine due to its highest electronegativity (and only –1 oxidation state) does not form polyhalide ions where it acts as a central atom.