The formula for the iodide ion is written as an I followed by the superscript -1. Iodine has several possible oxidation states of 7, 5, 1 and -1. However, the iodide ion indicates the -1 charge.
Ionic compounds form hard crystals. They dissolve in water to create electrical conductors. However, in their solid state, the crystalline structure of the ionic compound makes it a good insulator. Potassium iodide, written as KI, is one of the ionic compounds possible with the iodide ion. Producers often add potassium iodide to table salt, another ionic compound, to ensure consumers receive adequate amounts of this nutrient in their diet.