The choice of using 1/12th of the mass of a carbon-12 atom as the standard for relative atomic mass is based on historical convention and practical considerations.
The concept of relative atomic mass emerged from the need to compare the masses of different atoms. Initially, scientists used hydrogen as the reference point and assigned it a mass of 1. However, it was later discovered that hydrogen has different isotopes with different masses, and assigning it a fixed value became problematic.
To overcome this, scientists sought a more stable and common reference. Carbon-12, which has 6 protons and 6 neutrons, was chosen because it is the most abundant and stable isotope of carbon. Carbon-12 was assigned an atomic mass of exactly 12 atomic mass units (u). The choice of carbon-12 as the reference isotope allows for convenient comparisons between different elements.
Using carbon-12 as the standard, the relative atomic masses of other elements are determined by comparing their masses to the mass of carbon-12. For example, an element with an atomic mass of 24 u would be twice as heavy as carbon-12.
The choice of 1/12th of the mass of a carbon-12 atom is practical because it allows for convenient whole number values for atomic masses. It is a consistent and widely accepted convention that simplifies calculations and the representation of atomic masses in a concise manner.