Science, Tech, Math › Science Moiety Definition in Chemistry Share Flipboard Email Print ollaweila / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate by Anne Marie Helmenstine, Ph.D. Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. Updated September 04, 2019 In chemistry, a moiety is a specific group of atoms within a molecule that is responsible for characteristic chemical reactions of that molecule. Although sometimes the terms moiety and functional group are interchanged, a functional group is a smaller group of atoms. Moieties are branches in organic molecules that extend from the carbon backbone. Often, moieties may be substituted with other substituents or side chains. In pharmacology, an active moiety is the portion of an ion or molecule responsible for the activity of a drug. Examples: hydroxyl moiety: -OHaldehyde moiety: -COH Continue Reading