We use atomic mass units (AMU) to measure atomic mass because it provides a convenient and standardized unit for expressing the mass of atoms and molecules.
The atomic mass of an atom is determined by the combined mass of its protons, neutrons, and electrons. However, the mass of an electron is extremely small compared to the masses of protons and neutrons, so it is often neglected when calculating atomic mass.
The AMU is defined as one-twelfth the mass of a carbon-12 atom. Carbon-12 was chosen as the reference because it is a common element found in organic compounds and is stable with six protons and six neutrons. By comparing the mass of other atoms to the mass of a carbon-12 atom, we can express atomic masses as relative values.
Using AMU allows scientists to work with manageable numbers when dealing with extremely small masses, such as those of individual atoms. It provides a scale that allows for easy comparison of the masses of different atoms and elements.