Rb, or rubidium, is a chemical element with the symbol Rb and atomic number 37. It is a soft, silvery-white metallic element that belongs to the alkali metal group in the periodic table. Rubidium is highly reactive and is found in trace amounts in minerals such as lepidolite and carnallite. Due to its reactivity, Rb is typically stored in inert gases or under oil to prevent it from reacting with moisture in the air. It has applications in various fields, including atomic clocks, fireworks, and research in quantum mechanics.