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A unit of mass used to express atomic and molecular weight, equal to one twelfth of the mass of an atom of carbon-12 One atomic mass unit (u) is defined as the 1/12 th of the mass of the isotope of carbon 𝐴 1 2 6 𝐴 2 1 2 2 6 C. Even though atoms are very tiny pieces of matter, they have mass. Their masses are so small, however, that chemists often use a unit other than grams to express them—the atomic mass unit. The atomic mass unit (abbreviated u, altho ugh amu is a lso used) is defined as 1/12 of the mass of a 12C atom: It is equal to 1.661 × 10 −24 g. Learn what 1 atomic mass unit (AMU) is equal to in different units of mass, such as kilograms, grams, electron volts, and daltons. Find out why AMU is important in chemistry and physics and how it is used to calculate molecular masses.
