Science, Tech, Math › Science Atomic Mass Definition: Atomic Weight What Is Atomic Mass? Print Atomic mass or weight is the average mass of the protons, neutrons, and electrons in an element's atoms. Science Photo Library - ANDRZEJ WOJCICKI, 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. 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. Learn about our Editorial Process Updated on December 07, 2019 Atomic Mass or Weight Definition Atomic mass, which is also known as atomic weight, is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally occurring element. Read More Atomic Weight Definition By Anne Marie Helmenstine, Ph.D. Atomic mass indicates the size of an atom. Although technically the mass is the sum of the mass of all the protons, neutrons, and electrons in an atom, the mass of an electron is so much less than that of the other particles, that the mass is simply that of the nucleus (protons and neutrons). Examples of Atomic Mass The atomic mass of carbon is 12.011. Most carbon atoms consist of six protons and six neutrons. The atomic mass of hydrogen is 1.0079. Hydrogen (atomic number 1) is the element that has the lowest atomic mass. The most common isotope of hydrogen is protium, an atom that consists of a proton or a proton and an electron. Because of a small amount of deuterium (one proton and one neutron) and tritium (one proton and two neutrons), the atomic mass of hydrogen is slightly higher than 1. How to Calculate Atomic Mass Cite this Article Format mla apa chicago Your Citation Helmenstine, Anne Marie, Ph.D. "Atomic Mass Definition: Atomic Weight." ThoughtCo, Aug. 26, 2020, thoughtco.com/definition-of-atomic-mass-weight-604375. Helmenstine, Anne Marie, Ph.D. (2020, August 26). Atomic Mass Definition: Atomic Weight. Retrieved from https://www.thoughtco.com/definition-of-atomic-mass-weight-604375 Helmenstine, Anne Marie, Ph.D. "Atomic Mass Definition: Atomic Weight." ThoughtCo. https://www.thoughtco.com/definition-of-atomic-mass-weight-604375 (accessed April 24, 2024). copy citation