The relative atomic mass of carbon-12 is defined as exactly 12 because it serves as the reference point for the atomic mass scale. The atomic mass scale is based on the mass of carbon-12, which is assigned a value of exactly 12 atomic mass units (AMU).
The decision to assign carbon-12 a mass of 12 AMU was made to simplify the calculations and comparisons of atomic masses. Carbon-12 was chosen because it is a stable and abundant isotope of carbon, with six protons and six neutrons.
Since the atomic mass of carbon-12 is defined as exactly 12, it provides a consistent and reliable reference point for comparing the masses of other atoms and elements. The atomic masses of other elements are determined by measuring the mass of atoms relative to the mass of carbon-12 through various experimental techniques, such as mass spectrometry.
It's important to note that the relative atomic masses of other elements are not whole numbers because they are calculated based on the average mass of the naturally occurring isotopes of each element, which have different masses due to varying numbers of neutrons.