monatomic
adjective
mon·atom·ic ˌmä-nə-ˈtä-mik 
: consisting of one atom
especially : having but one atom in the molecule
Example Sentences
Recent Examples on the Web In particular, both diatomic hydrogen and monatomic helium can permeate the seals. 
Peter Bright, Ars Technica, 31 Oct. 2018
Peter Bright, Ars Technica, 31 Oct. 2018 Word History
First Known Use
1848, in the meaning defined above
Medical Definition
monatomic
adjective
mon·atom·ic ˌmän-ə-ˈtäm-ik
1
: consisting of one atom
especially : having but one atom in the molecule
2
: monovalent sense 1