Question

Define atomic mass unit
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Answer 1

AMU :

an atomic mass unit is defined as a mass equal to one twelfth of the mass of an atom of carbon 12

Answer 2

Atomic weight, also called relative atomic mass, ratio of the average mass of a chemical element’s atoms to some standard. Since 1961 the standard unit of atomic mass has been one-twelfth the mass of an atom of the isotope carbon-12. An isotope is one of two or more species of atoms of the same chemical element that have different atomic mass numbers (protons + neutrons). The atomic weight of helium is 4.002602, the average that reflects the typical ratio of natural abundances of its isotopes