When potassium (K) and iodine (I) react, they form potassium iodide (KI). The reaction can be represented as follows:

$$2K(s) + I_2(s) → 2KI(s)$$

In the reaction, potassium atoms lose one valence electron each to become potassium ions (K+). Iodine atoms gain one valence electron each to become iodide ions (I-). The potassium and iodide ions then combine to form potassium iodide, a white, crystalline solid.