Carbon-12 (C-12) is chosen as the reference isotope for Avogadro's number and the atomic mass unit (amu) because it has several favorable properties that make it suitable for this purpose. Here are the main reasons:
Abundance: Carbon is a common element found in the Earth's crust, atmosphere, and living organisms. Carbon-12 is the most abundant stable isotope of carbon, making up about 98.9% of naturally occurring carbon. Its abundance ensures that it is readily available for research and experimentation.
Stability: Carbon-12 is a stable isotope, meaning it does not undergo radioactive decay over practical timescales. This stability allows for accurate and consistent measurements of its properties.
Atomic mass: The atomic mass of carbon-12 is defined as exactly 12 atomic mass units (amu). The amu is a unit of mass used to compare the masses of atoms and molecules. By defining the atomic mass of carbon-12 as exactly 12 amu, it provides a convenient and consistent reference point for determining the atomic masses of other elements relative to carbon-12.
Relative atomic mass scale: The choice of carbon-12 as the reference isotope is tied to the development of the relative atomic mass scale. The relative atomic mass of an element is determined by comparing the mass of its atoms to the mass of a carbon-12 atom. This scale allows scientists to express atomic masses in a consistent and meaningful way.
By choosing carbon-12 as the reference isotope, Avogadro's number (which represents the number of atoms or molecules in one mole of a substance) can be determined based on accurate measurements of carbon-12 and its relationship to the atomic mass unit. This choice provides a foundation for the quantitative study of chemistry and allows for precise calculations and comparisons of quantities at the atomic and molecular scale.