Carbon-12 (12C) is taken as a standard for calculating relative atomic mass due to several reasons:
Abundance: Carbon-12 is one of the most abundant isotopes of carbon, making up around 98.9% of naturally occurring carbon. This high abundance ensures that measurements of carbon-12 are readily available and accurate.
Stability: Carbon-12 is a stable isotope, meaning it does not undergo radioactive decay. Its stability allows for precise and consistent measurements over time.
Mass: Carbon-12 has a conveniently round mass of exactly 12 atomic mass units (u) on the unified atomic mass scale. This makes calculations and comparisons easier when determining the relative atomic masses of other elements.
Choice of scale: The choice of carbon-12 as the standard was made to establish a scale where 12 atomic mass units (u) correspond to the mass of one carbon-12 atom. This scale allows for the representation of relative atomic masses in a consistent and meaningful way.
By taking carbon-12 as the standard, scientists have established a reference point for comparing the masses of other atoms and isotopes. The relative atomic masses of elements are determined by comparing them to the mass of carbon-12, which simplifies calculations and provides a consistent basis for understanding the atomic masses of other elements.