For determining atomic mass, which of the following is selected as a standard reference?

Correct option is A

The correct answer is (A) Carbon-12

Explanation:
• In 1961, the International Union of Pure and Applied Chemistry (IUPAC) globally adopted the Carbon-12 isotope as the universal standard reference for defining atomic mass.
• An atomic mass unit (amu or simply 'u') is officially defined as a mass equal to exactly one-twelfth (1/12th) of the mass of a single carbon-12 atom.
• Carbon-12 was chosen because it is abundant, highly stable, and allows the atomic masses of most other elements to be expressed as whole numbers or very close to whole numbers.

Information Booster:
• Historical Standards: Before Carbon-12 was adopted, physicists used Oxygen-16 as a standard, while chemists used naturally occurring oxygen (a mixture of isotopes) as the standard, causing significant scientific confusion.
• Isotopes: Carbon-12 has 6 protons and 6 neutrons. It accounts for about 98.9% of all natural carbon on Earth.

Additional Knowledge:
• Option B (Carbon-13): A heavier, stable isotope of carbon making up about 1.1% of natural carbon. It is extensively used in NMR spectroscopy but is not the mass standard.
• Option C (Nitrogen-14) & D (Nitrogen-15): These are stable isotopes of nitrogen. While important in agriculture and biological tracing, they have never been standard references for atomic mass scales.