What is the ionic charge when iodine forms an ion?

The element iodine will exist as an anion with a charge of -1.

Ions

An element can exist as an ion in order to form an ionic compound. It can exist as a cation (positively-charged ion) or an anion (negatively-charged ion). The element can be predicted if it will become a cation or anion by looking at the number of valence electrons.

The elements try to occupy the outer valence shell with 8 electrons so they tend to accept or lose an electron to achieve the octet rule (8 valence electrons in the outer valence shell).