Concept:Atoms are incredibly tiny, making macroscopic weighing impossible. To solve this, scientists established a comparative scale where the mass of every element is weighed relative to one specific, highly stable atomic isotope.
Formula:$$ 1 \text{ a.m.u.} = \frac{1}{12} \times \text{Mass of one } ^{12}\text{C atom} $$
Solution:- In 1961, IUPAC internationally designated the Carbon-12 isotope as the absolute standard baseline.
- The mass of one Carbon-12 atom is defined as exactly 12 atomic mass units.
- Therefore, the carbon atom is exclusively used as the standard benchmark to measure and assign the relative atomic mass of all other elements on the periodic table.
Why other options are incorrect:- Atomic size & volume: These are measured in picometers or Angstroms using X-ray crystallography, not by relative comparison to a carbon standard.
- Atomic number: This is an absolute integer count of protons (discovered via Moseley's law), requiring no relative elemental standard.
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