Iggy's Element Bonanza! Element: Rubidium Atomic Symbol: Rb Atomic Number: 37 Iggy: Rubidium is the next Alkali Metal under Potassium, thus making it even more reactive than Potassium. It is soft and can be cut with a knife lika all the other alkali metals. It form RbOH and H2 gas when reacted with water. Rubidium has one stable isotope, Rb-85 and Rb-87 is radioactive but occurs naturally.