Science, Tech, Math › Science Free Radical Definition Chemistry Glossary Definition of Free Radical Share Flipboard Email Print Healthvalue/Wikimedia Commons/CC 3.0 SA 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. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College 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. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated July 03, 2019 An atom or molecule with an unpaired electron. Because they have a free electron, such molecules are highly reactive. Examples Singlet oxygen, molecules with a free hydroxy group (-OH) Properties Free radicals are capable of starting rapid chain-reactions that destabilize the ions in other nearby molecules generating more free radicals. In biological systems, free radicals are deactivated by anti-oxidants, uric acid, and certain enzyme activities.