Concept:Because atoms are impractically light to weigh in standard grams, scientists created the Atomic Mass Unit (amu) to establish a convenient relative scale.
Formula:$$ 1 \text{ amu} = \frac{1}{12} \times \text{Mass of a single Carbon-12 atom} $$
Solution:- In 1961, IUPAC internationally defined one atomic mass unit as exactly one-twelfth (\( 1/12 \)) of the mass of an unbound neutral atom of carbon-12 at rest.
- Since a C-12 atom contains 6 protons and 6 neutrons, dividing its mass by 12 yields a mass roughly equivalent to a single generic nucleon (proton or neutron).
Why other options are incorrect:- An atom of C-12: This would make 1 amu equivalent to a mass of 12. It must be \( 1/12\text{th} \) of it.
- 1/12th of H: Hydrogen is far too light. \( 1/12\text{th} \) of Hydrogen would be less than a tenth of a proton, which is scientifically useless as a baseline standard.
- 1 atom of all the elements: A nonsensical statement, as every element has a different mass.
Quality & Fidelity Assurance:
Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.