Science, Tech, Math › Science How to Calculate Atomic Weight Share Flipboard Email Print EzumeImages/Getty Images Science Chemistry Basics Chemical Laws 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 June 27, 2019 The atomic weight of an element depends on the abundance of its isotopes. If you know the mass of the isotopes and the fractional abundance of the isotopes, you can calculate the element's atomic weight in atomic mass units (expressed as u, Da, or amu). The atomic weight is calculated by adding the mass of each isotope multiplied by its fractional abundance. For example, for an element with 2 isotopes: atomic weight = massa x fracta + massb x fractb If there were three isotopes, you would add a 'c' entry. If there were four isotopes, you'd add a 'd', etc. Atomic Weight Calculation Example If chlorine has two naturally-occurring isotopes where: Cl-35 mass is 34.968852 and fract is 0.7577Cl-37 mass is 36.965303 and fract is 0.2423 atomic weight = massa x fracta + massb x fracb atomic weight = 34.968852 x 0.7577 + 36.965303 x 0.2423 atomic weight = 26.496 amu + 8.9566 amu atomic weight = 35.45 amu Tips for Calculating Atomic Weight The sum of the fractional abundance values must equal 1.Be sure to use the mass or weight of each isotope and not its mass number.