The use of atomic mass units (u) instead of yoctograms (10^-24 grams) to describe atomic mass is primarily due to historical reasons and convenience within the field of atomic and nuclear physics.
The atomic mass unit (u) was introduced as a unit specifically designed to express the masses of atoms and atomic-scale particles. It is based on the relative mass of a carbon-12 atom, which was chosen as a reference point. The atomic mass unit is widely used and accepted within the scientific community working in the field of atomic and nuclear physics.
While the SI unit for mass is the kilogram, which is the base unit of mass in the International System of Units, using yoctograms (10^-24 grams) to express atomic mass would result in extremely small numbers for most atoms. This can lead to impractical and cumbersome calculations and representations.
Furthermore, atomic masses are relative quantities, and the use of atomic mass units simplifies comparisons and calculations involving atomic masses. It provides a convenient scale for expressing the mass of different atoms and isotopes relative to each other.
It's important to note that atomic mass units are widely recognized and accepted within the scientific community as a standard unit for atomic masses. However, in certain contexts, you may find atomic masses expressed in other units, such as kilograms or grams, depending on the specific requirements of the field or application.