We know that atoms are very small units that combine to form molecules. Because of their small size, atoms are not countable units. Therefore we must have some measurement criteria to measure the mass of an atom or molecule. We have to determine the number of atoms in a molecule to calculate the molecular mass. The first method was purposed by John Dalton and Thomson to determine the atomic weights of atoms. We cannot calculate the atomic weight directly by taking that atom, but we have to take the weight of an atom relative to some atom. For example; hydrogen is the lightest element with atomic weight 1.00. Similarly we can consider 1/16th of mass of 1 atom of O as a unit of atomic mass.
Today we are using the carbon element as standard element for the measurement of atomic mass whose atomic mass is considered as 12 amu. Hence, 1 atomic mass unit is equal to 1/12th of the mass of one atom of C-12. The unit of atomic mass is equal to the mass of one H-atom. The relative weights of H, C and Nare considered as 1.01, 12.01 and 14.01. We can check the atomic mass of other elements with respect to these standard elements.