Question

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

 

Answer 1

• Since the atom is very very small it is very difficult to determine its mass very accurately, Hence the concept of the relative mass was
• As the hydrogen atom (H) is the lightest it was taken as the reference atom in old The nucleus of H atom contains only one proton, therefore its mass was taken as one unit.
• The mass of nitrogen atom (N) is 14 times than that of the H atom. Hence the relative mass of the N atom was taken as 14 units. In this manner the relative masses of various elements were determined.
• In 1961 it was decided to take the C-12 atom as the reference atom. In this the mass of C-12 atom is taken as 12 u. The mass of the hydrogen atom relative to that of the C-12 atom is equal to 1/12 x 12 u is equal to 1 u. (Note: 1 u =
• 1.66053904 x 10^-27 kg ).
• On the relative atomic masS of scale, the mass of the proton is very nearly 1 u. Similarly the mass of the neutron is very nearly 1 u. (Note : The mass of the neutron is slightly greater than that of the)