Question

The modern atomic weight scale is based on
(A) ${C^{12}}$
(B) ${O^{16}}$
(C) ${H^1}$
(D) ${C^{13}}$

Answer 1

The standard unit for expressing the mass of an atom is amu (atomic mass unit). One amu is defined as the mass of an atom which is equal to $1/12$ of the mass of an atom of carbon- $12$ . The mass of any isotope of any element is expressed in relation to the carbon- $12$ standard.

Complete step by step solution:
The carbon- $12$ atom has six protons and six neutrons in its nucleus for a mass number of $12$ . Since the nucleus accounts for nearly all the mass of the atom (electrons have negligible mass), a single proton or a single neutron has a mass of approximately $1$ amu.
The reason why carbon- $12$ is taken as standard for atomic mass is because no other nuclides other than carbon- $12$ have exactly whole number masses in this scale. This is due to two factors:
$1)$ the different masses of neutrons and protons acting to change the total mass in nuclides with proton $/$ neutron ratio other than the $1:1$ ratio of carbon- $12$ .
$2)$ an exact whole number will not be located if there exists a loss $/$ gain of mass to difference in mean binding energy relative to the mean binding energy for carbon- $12$ .
Keeping these factors in mind, carbon- $12$ was selected as the standard for modern atomic weight scale.
So, the correct option will be (A): ${C^{12}}$ .

Additional Information
Earlier, oxygen was used as standard but the presence of two isotopes (oxygen- $17$ and oxygen- $18$ ) lead to two different tables of atomic masses. Hence, it was replaced by carbon- $12$ .

Note:
In $1961$ , the isotope carbon- $12$ was selected to replace oxygen as the standard relative to which atomic weights of all other elements are measured. Carbon- $14$ is radioactive and this isotope is used in radioactive dating and radiolabeling.