Iggy's Element Bonanza! Element: Potassium Atomic Symbol: K Atomic Number: 19 Iggy: Potassium is the third member of the Alkali Metals family, it is more reactive than Sodium and can explode when put into water! Like Sodium and Lithium, Potassium is soft and can be easily cut. Potassium burns with a lilac flame. Potassium has 2 stable isotopes, K-39 and K-41 and the long lived radioisotope, K-40!