Mass of atom is called atomic mass. Since, atoms are very small consequently actual mass of an atom is very small. For example the actual mass of one atom of hydrogen is equal to 1.673 x 10-24 g. This is equal to 0.000000000000000000000001673 gram. To deal with such small number is very difficult. Thus for convenience relative atomic mass is used.
Carbon-12 is considered as unit to calculate atomic mass. Carbon-12 is an isotope of carbon.
The relative mass of all atoms are found with respect to C-12.
One atomic mass = 1/12 of the mass of one atom of C-12.
Thus atomic mass is the relative atomic mass of an atom with respect to 1/12 th of the mass of carbon-12 atom. ‘amu’ is the abbreviation of Atomic mass unit, but now it is denoted just by ‘u’.
The atomic mass of hydrogen atom = 1u.
This means one hydrogen atom is 1 times heavier than 1/12th of the carbon atom.
The atomic mass of oxygen is 16u, this means one atom of oxygen is 16 times heavier than 1/12th of carbon atom.
Absolute mass or Actual atomic mass:
It is found that, the actual atomic mass of a carbon-12 atom is equal to 1.9926 X 10-23g.
Thus by multiplying the relative atomic mass with 1.6605 × 10-24g we can get the absolute or actual mass of an atom.
