Question

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

Answer 1

Hint: 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.