Science, Tech, Math › Science Definition of Alpha Decay Share Flipboard Email Print C. POWELL, P. FOWLER & D. PERKINS, 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. 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 February 06, 2018 Alpha decay is the spontaneous radioactive decay where an alpha particle is produced. An alpha particle is essentially a helium nucleus or He2+ ion. Although alpha decay presents a significant radiation risk if the radioactive source is inhaled or ingested, alpha particles are too large to penetrate very far through the skin or other solids and require minimal radiation shielding. A sheet of paper, for example, blocks alpha particles.An atom that undergoes alpha decay will reduce its atomic mass by 4 and become the element two atomic numbers less. The general reaction to alpha decay isZXA → Z-4YA-2 + 4He2where X is the parent atom, Y is the daughter atom, Z is the atomic mass of X, A is the atomic number of X. Examples: 238U92 decays by alpha decay into 234Th90.