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Occurring as single atoms: Helium is a monatomic gas.
1. (Chemistry) (of an element) having or consisting of single atoms: argon is a monatomic gas.
2. (Chemistry) (of a compound or molecule) having only one atom or group that can be replaced in a chemical reaction
3. (Chemistry) a less common word for monovalent
mon•a•tom•ic(ˌmɒn əˈtɒm ɪk)
1. having one atom in the molecule.
2. containing one replaceable atom or group.
3. having a valence of one.