The noble gases, including radon, helium, argon, xenon, neon and krypton, are all monatomic. Since the noble gases are the only stable gases, they're always monatomic. However, if the temperature is high enough, other gases on the periodic table can become monatomic, such as hydrogen.
Monatomic gases are those without atoms that bond together; instead, their particles are single atoms. The stability of these gases depends on the pressure and temperature, and the noble gases may be reactive or non-reactive depending on how heavy they are.
Some other types of gases include diatomic gases, which have two atoms, and polyatomic gases, which are much more complex with many atoms.