Entry 1 sense Webster, 1913 Monatomic /mänˌ-ət-äm'-ĭk/ · Mon·a·tom·ic · IPA /mˌɑnətˈɑmɪk/ 01 adv. Consisting of, or containing, one atom; as, the molecule of mercury is monatomic. 1. Consisting of, or containing, one atom; as, the molecule of mercury is monatomic.(Chem.)