Rubidium is an element, meaning it's a pure substance that cannot be broken down into simpler substances by chemical means. So, rubidium is made of rubidium atoms.

Here's a bit more detail:

* Atoms: Rubidium atoms consist of a nucleus containing protons and neutrons, surrounded by a cloud of electrons.

* Protons: The number of protons in an atom defines the element. Rubidium has 37 protons.

* Neutrons: Rubidium can have different numbers of neutrons, creating isotopes. The most common isotope is rubidium-85, which has 48 neutrons.

* Electrons: Electrons orbit the nucleus in shells, and their arrangement determines the chemical properties of the element. Rubidium has 37 electrons.

So, to put it simply, rubidium is made of rubidium atoms, which are in turn made of protons, neutrons, and electrons.