Carbon-12 is commonly used as the standard for measuring relative atomic mass because it has several desirable properties that make it a suitable choice for this purpose. The decision to use carbon-12 as the standard is based on practical and historical considerations.
One of the main reasons carbon-12 is chosen is its abundance and stability. Carbon is the fourth most abundant element in the universe and is found in a wide range of natural materials. Carbon-12 is the most abundant and stable isotope of carbon, making it readily available and consistent in its properties. This abundance and stability ensure that measurements based on carbon-12 are reliable and reproducible.
Another important factor is the relative atomic mass scale itself. The relative atomic mass scale is based on a comparison to the mass of the carbon-12 atom, which is assigned a mass of exactly 12 atomic mass units (amu). This scale was established long before the understanding of isotopes, and carbon-12 was chosen because its atomic mass is close to 12 amu, making the arithmetic convenient.
Hydrogen, specifically the protium isotope, could have been used as an alternative standard. However, there are a few reasons why carbon-12 is preferred over hydrogen. First, hydrogen has three naturally occurring isotopes: protium, deuterium, and tritium. These isotopes have different masses, and using hydrogen as a standard would require specifying which isotope is being referred to. This would complicate the measurement and reporting of atomic masses.
Additionally, hydrogen is the lightest element, and its atomic mass is very close to 1 amu. Using hydrogen as the standard would result in atomic masses of other elements being much higher and less convenient to work with.
Overall, carbon-12 is a practical choice for a standard because of its abundance, stability, and its proximity to a mass of 12 amu, making it a widely accepted and convenient reference point for measuring relative atomic masses.