mon·atomic \|män, |mōn+\adjective Etymology:mon- + atomic 1. a.: consisting of one atom : having one atom in the molecule : atomic 5 < helium is a monatomic gas > < monatomic metals > b.: having a thickness equal to the diameter of a constituent atom < a monatomic layer of cesium > 2.: univalent 3.: having one replaceable atom or radical < monatomic alcohols >