monatomic

adj. having one atom (Chemistry)

In physics and chemistry, monatomic is a combination of the words "mono" and "atomic," and means "single atom." It is usually applied to gases: a monatomic gas is one in which atoms are not bound to each other.Under Standard Temperature and Pressure (STP), all of the noble gases are monatomic. These are helium, neon, argon, krypton, xenon and radon. The heavier noble gases can form compounds, but the lighter ones are unreactive. All elements will be monatomic in the gas phase at sufficiently large temperatures.
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Adjective
1. of or relating to an element consisting of a single atom; "helium and argon are monatomic gases"
(synonym) monoatomic
(pertainym) atom

(adv.)
Having the equivalence or replacing power of an atom of hydrogen; univalent; as, the methyl radical is monatomic.
  

(adv.)
Consisting of, or containing, one atom; as, the molecule of mercury is monatomic.
  

MONOATOMICO