Question

The modern atomic mass unit is based on the mass of:
(A) C-12 isotope
(B) Hydrogen
(C) Oxygen
(D) Nitrogen

Answer 1

We know that the atomic mass of a given atom is the actual physical mass of the atom. The numerical value of the atomic mass is very small. The SI unit of atomic mass is kilograms. We have set a reference and the atomic mass of the elements has been defined with respect to that reference.

Complete step by step solution:
We know that the atomic mass of an atom is the actual mass of an atom. The SI unit of atomic mass is kilograms. In non-SI units, atomic mass is represented as Dalton. 1 Dalton is defined as $\dfrac{1}{{12}}{\text{th}}$ of the mass of a single carbon-12 atom.
The nucleus of an atom consists of protons and neutrons and the nucleus accounts for the total mass of an atom. The electrons and the nuclear binding energy make minor contributions to the total mass of an atom.
When atomic mass is expressed in Dalton, the numerical value of the atomic mass is equal to the mass number. One atomic mass unit is equal to ${1.6605389210^{ - 24}}{\text{ g}}$ or ${1.6605389210^{ - 27}}{\text{ kg}}$. One atomic mass unit is equal to $\dfrac{1}{{12}}{\text{th}}$ of the mass of a single carbon-12 atom.
The atomic mass unit is expressed as follows:
${m_u} = \dfrac{{m\left( {^{{\text{12}}}{\text{C}}} \right)}}{{12}} = 1{\text{ Da}}$
Where ${m_u}$ is the molar mass constant,
$m\left( {^{{\text{12}}}{\text{C}}} \right)$ is the molar mass of the carbon-12 atom.
Thus, the modern atomic mass unit is based on the mass of ${\text{C}} - 12$ isotope.

Thus, the correct option is (A) ${\text{C}} - 12$ isotope.

Note: Carbon-12 contains six protons, six neutrons and six electrons. Carbon-12 is the most abundant isotope of carbon. The atomic mass of carbon-12 is exactly $12{\text{ daltons}}$. The abundance of carbon-12 is due to the triple-alpha cycle. The numerical values of atomic mass are very small.