Science, Tech, Math › Science Atomic Mass Versus Mass Number Atomic Mass and Mass Number Don't Mean the Same Thing Share Flipboard Email Print ThoughtCo / Kaley McKean 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 Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated February 04, 2020 There is a difference between the meanings of the chemistry terms atomic mass and mass number. One is the average weight of an element and the other is the total number of nucleons in the atom's nucleus. Atomic mass is also known as atomic weight. Atomic mass is the weighted average mass of an atom of an element based on the relative natural abundance of that element's isotopes.The mass number is a count of the total number of protons and neutrons in an atom's nucleus. Key Takeaways: Atomic Mass Versus Mass Number The mass number is the sum of the number of protons and neutrons in an atom. It is a whole number.The atomic mass is the average number of protons and neutrons for all natural isotopes of an element. It is a decimal number.Atomic mass value sometimes change over time in publications as scientists revise the natural isotope abundance of elements. Atomic Mass and Mass Number Example Hydrogen has three natural isotopes: 1H, 2H, and 3H. Each isotope has a different mass number. 1H has 1 proton; its mass number is 1. 2H has 1 proton and 1 neutron; its mass number is 2. 3H has 1 proton and 2 neutrons; its mass number is 3. 99.98% of all hydrogen is 1H. It is combined with 2H and 3H to form the total value of atomic mass of hydrogen, which is 1.00784 g/mol. Atomic Number and Mass Number Be careful you don't confuse atomic number and mass number. While the mass number is the sum of the protons and neutrons in an atom, the atomic number is only the number of protons. The atomic number is the value found associated with an element on the periodic table because it is the key to the element's identity. The only time the atomic number and mass number are the same is when you are dealing with the protium isotope of hydrogen, which consists of a single proton. When considering elements in general, remember the atomic number never changes, but because there may be multiple isotopes, the mass number may change. View Article Sources Klein, David R. Organic Chemistry. 3rd ed., John Wiley & Sons, Inc., 2017.