Carbon-12 was selected as the standard element for atomic mass primarily because of its abundance, stability, and the ability to accurately determine its atomic mass.
The concept of atomic mass is based on the relative masses of atoms compared to a standard reference. In the early 20th century, scientists were developing a standardized method for measuring atomic masses to establish a consistent scale. Carbon, being a common element found in organic compounds and readily available, was chosen as the basis for this scale.
Among the isotopes of carbon, carbon-12 was specifically selected because it is the most abundant and stable isotope. It has six protons, six neutrons, and six electrons, giving it a total atomic mass of 12 atomic mass units (amu). This made it convenient to assign a mass of exactly 12 amu to carbon-12.
Carbon-12 isotope was also chosen because it has a low relative isotopic variation, meaning that the differences in mass between different samples of carbon-12 are very small. This consistency makes it easier to determine and reproduce accurate measurements of atomic masses.
Using carbon-12 as the standard reference allowed scientists to establish a unified scale for atomic masses. The atomic mass unit (amu) is defined as 1/12th the mass of a carbon-12 atom, providing a consistent and universally accepted standard for measuring the masses of other atoms and molecules.