CBSE Class 9 Answered

Atoms being extremely small cannot be seen or weighed directly.

But indirect methods of physics have enabled us to determine the absolute mass of nearly all kinds of atoms.

The mass of hydrogen (1.6735 x 10^-24g) being the lightest was chosen and masses of other atoms were compared with it.

The resulting masses of atoms are thus relative atomic masses.

Later on carbon-12 was finally selected because its adoption least affected the values of the atomic mass of the various elements on the old standard.

The relative atomic mass of an element is the number of times one atom of the element is heavier than 1/12^th times of the mass of an atom of carbon-12.

Relative atomic mass = Mass of 1 atom of the element

                                   1/12^th the mass of one C-12 atom

Atomic mass unit is expressed in a.m.u