10.

What universal standard is the basis of the atomic mass unit?

the chlorine-35 atom

the proton

the oxygen-16 atom

the carbon-12 atom

Answer: the carbon-12 atom

Explanation: The International Union of Pure and Applied Chemistry (IUPAC), defined 1 atomic mass unit as 1/12 the mass of carbon-12. That is to say carbon-12 has a mass of 12 amu. The carbon-12 isotope is abundant in nature. Carbon is usually found as a compound with other elements.

After many years of debating, chemists and physicists agreed to the use of carbon-12 to be the best unit for the measurement for atomic masses.

Previous standards that was used were hydrogen and oxygen but scientists found it difficult using these elements as standards due to the distribution of their isotopes.

The answer would be D, the carbon-12 atom.