The relative atomic masses of all elements are determined with respect to an atom of carbon-12.
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One atomic mass unit is defined as exactly one-twelfth (1/12th) the mass of one atom of carbon-12.
Scientists initially determined the atomic mass unit for oxygen by taking 1/16 of the mass of an atom of naturally occurring oxygen as the unit.
Expressing atomic masses as whole numbers or close to whole numbers makes calculations and comparisons simpler and more intuitive.
Carbon was assigned an atomic mass of 1.0 u, and oxygen was assigned an atomic mass of 1.33 u.