Q. 4 C

Write answers to the following questions and explain your answers.

Explain the need for a reference atom for atomic mass. Give some information about two reference atoms.

Answer :

It was not possible for scientists of the 19th century to measure atomic mass accurately. Therefore, the concept of ‘relative mass of an atom’ was put forth. A reference atom was required for expressing the relative mass of an atom. The hydrogen atom is the lightest was initially chosen as the reference atom. The relative mass of a hydrogen atom which has only one proton in its nucleus was accepted as one (1).

Therefore, the magnitude of the relative atomic masses of various atoms became equal to their atomic mass number (p + n).


In the course of time, different atoms were chosen as reference


atoms. Finally in 1961, the carbon atom was selected as the reference atom. In this scale, the relative mass a carbon atom was accepted as 12. The relative atomic mass of one hydrogen


atom compared to the carbon atom becomes 12 x 1/12, that is 1. The mass of one proton and of one neutron on the scale of relative atomic masses is approximately one.


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