The atomic mass unit (amu) is a unit of mass commonly used in atomic and molecular physics to express atomic and molecular weights. It is defined as 1/12th the mass of a carbon-12 atom. The use of atomic mass units is convenient because it allows for a unified scale to express the relative masses of atoms and molecules.
While it is true that you can calculate the average atomic mass of an element by considering the abundance percentages and mass numbers of its isotopes, the atomic mass unit provides a standardized and convenient way to express these masses on a numerical scale.
The atomic mass unit allows scientists to compare the masses of different atoms and molecules and make meaningful calculations involving stoichiometry, molar masses, and other important concepts in chemistry and physics. It simplifies calculations and facilitates the understanding of atomic and molecular properties.
Additionally, it's important to note that the atomic mass unit is a scale based on the carbon-12 isotope. The atomic masses listed in the periodic table are weighted averages of the masses of naturally occurring isotopes, taking into account their relative abundances. The atomic mass unit provides a consistent and universal framework for expressing these masses.