Does one atom make a compound?
No, in order to be a compound it has to be two or more atoms
The ion for Potassium has a charge of 1+. The ion for Iodine has a charge of 1-. This means that in order to make the overall charge of a unit of a compound of Potassium and Iodine, there must be one atom of Potassium and one atom of Iodine. Therefore, when K+ and I- are bonded, they make the ionic compound of KI.