Iodine, when it forms an ion, typically gains an electron to achieve a full outer electron shell, similar to the noble gases. This process transforms a neutral iodine atom, which has seven valence electrons, into an iodide ion.
By gaining one electron, iodine increases its total number of electrons to eight. Since gaining an electron results in a negative charge, the charge of the iodide ion is -1. Therefore, the chemical representation of this ion is I–.
This behavior is typical for halogens in group 17 of the periodic table, as they tend to gain one electron to attain greater stability. In practical terms, this means that in ionic compounds, iodine commonly exists in an ionic form with a -1 charge.