Rubidium

chemistry · alkali metals · radioactive · elements

Softer than butter. Rubidium is so soft you can cut it with a knife, and it's whitish-grey like tarnished silver. It's one of the softest metals on Earth.

Sinks in water. Rubidium is the first alkali metal that is denser than water, which means it actually sinks instead of floating. This makes it unusual compared to its alkali metal cousins.

Mostly stable isotopes. About 72% of all natural rubidium is a stable form that never changes, while 28% is slightly radioactive. The radioactive form takes 48.8 billion years to break down, which is more than three times the age of the entire universe!

Family of metals. Rubidium belongs to the alkali metal group on the periodic table, alongside potassium and caesium. These metals all share similar properties and behaviors.