Rubidium, like most alkali metals, is monatomic. This means that rubidium exists as individual atoms, not as molecules.

Here's why:

* Alkali metals are highly reactive: They readily lose their single valence electron to form a +1 cation.

* Bonding with other rubidium atoms is unfavorable: Rubidium atoms have a very weak attraction to each other.

* Stability as individual atoms: Rubidium atoms are more stable as individual entities than as bonded molecules.

Therefore, when you encounter rubidium, it's in the form of individual rubidium atoms.