Which element's atomic mass is used as the standard for defining an atomic mass unit?

The atomic mass unit (amu) is defined based on the mass of carbon-12. Specifically, one atomic mass unit is defined as one twelfth of the mass of a carbon-12 atom. This choice is significant because carbon-12 is a stable isotope, and using it as a reference point allows for a standardized way to express the masses of other elements. The decision to use carbon-12 is historical and practical; carbon is a fundamental building block of life, and its isotopes are commonly found in nature.

Using carbon-12 provides a consistent scale for the atomic masses of all elements, making it easier to compare their relative masses. Other isotopes, such as carbon-14 or oxygen-16, while they can represent an atomic mass, do not serve as the standard definition for atomic mass units. Hydrogen, being the lightest element, also has a relatively simple mass, but the universal standard remains carbon-12 for its stability and abundance in natural processes.