Atomic symbol: Rb
Atomic number: 37
Atomic mass: 85.47
Appearance: Gray white solid at 25°C
Series: Alkali metals
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 5s¹
Very soft and extremely reactive: exposed to oxygen, rubidium reacts vigorously and exothermically, and may even catch fire.
Descending Group 1 from lithium to francium, rubidium is the first alkali metal that sinks in water because it has a greater density: lithium, sodium, and potassium all float.
Fun Fact! Rubidium is used in fireworks to give a purple color.