Science, Tech, Math › Science Avogadro's Number: Definition Share Flipboard Email Print 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. 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 January 23, 2020 Avogadro's number, or Avogadro's constant, is the number of particles found in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12. This experimentally determined value is approximately 6.0221 x 1023 particles per mole. Avogadro's number may be designated using the symbol L or NA. Note that Avogadro's number, on its own, is a dimensionless quantity. In chemistry and physics, Avogadro's number usually refers to a number of atoms, molecules, or ions, but it can be applied to any "particle." For example, 6.02 x 1023 elephants are the number of elephants in one mole of them! Atoms, molecules, and ions are much less massive than elephants, so there needed to be a large number to refer to a uniform quantity of them so that they could be compared relative to each other in chemical equations and reactions. History of Avogadro's Number Avogadro's number is named in honor of the Italian scientist Amedeo Avogadro. Although Avogadro proposed that the volume of a gas at a fixed temperature and pressure was proportional to the number of particles it contained, he did not propose the constant. In 1909, French physicist Jean Perrin proposed Avogadro's number. He won the 1926 Nobel Prize in physics for using several methods to determine the value of the constant. However, Perrin's value was based on the number of atoms in 1 gram-molecule of atomic hydrogen. Later, the constant was redefined based on 12 grams of carbon-12. In German literature, the number is also called the Loschmidt constant.