Concept:Relative atomic mass is a dimensionless physical quantity. It is defined as the ratio of the average mass of atoms of an element to exactly \( \frac{1}{12} \) of the mass of a single standard reference atom.
Formula:$$ A_r = \frac{\text{Average mass of one atom of the element}}{\frac{1}{12} \times \text{mass of one atom of } ^{12}\text{C}} $$
Solution:- Historically, Hydrogen and later Oxygen were used as standards.
- In 1961, the International Union of Pure and Applied Chemistry (IUPAC) established the Carbon-12 isotope as the universal standard.
- Because Carbon-12 is highly stable and abundant, it provides a consistent, universally accepted baseline (assigned an exact mass of 12.0000 amu) to compare the atomic masses of all other elements, including chlorine.
Why other options are incorrect:- Neon-20 & Carbon-13: These are stable isotopes but they are not the internationally recognized IUPAC standard.
- Nucleon number: This is an integer representing protons and neutrons, not a physical atomic mass standard.
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