Here's why:

* Noble gases are very stable: Their outer electron shells are completely filled, making them very unreactive. They don't readily form bonds with other atoms.

* Monatomic means existing as single atoms: Since noble gases are so stable, they exist as individual atoms rather than forming molecules.

Examples of noble gases:

* Helium (He)

* Neon (Ne)

* Argon (Ar)

* Krypton (Kr)

* Xenon (Xe)

* Radon (Rn)