Question

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

Answer 1

1. Mass of an atom is concentrated in its nucleus. It is the sum of proton(p) and neutron(n). The number (p + n) is called atomic mass number. 
2. Therefore, the concept of reference atom for atomic mass is taken into consideration.
   Initially hydrogen atom being lightest was chosen as the reference atom. The relative mass of hydrogen atom was accepted as (1). this is how relative atomic masses of various elements were determined in reference of hydrogen. For example: the mass of one nitrogen atom is fourteen (14) times that of a hydrogen atom.
3. In 1961, the carbon atom was selected as the reference atom. In this method, the relative mass of one atom of carbon was accepted as 12. The relative mass of a hydrogen atom compared to a carbon atom is `12 xx 1/12` i.e. 1 u.
4. On the relative masses of atoms, the mass of a proton and a neutron is approximately 1.