The atomic mass or the mass of an atom is actually very-very small because atoms are extremely small. Today, we have sophisticated techniques e.g., mass spectrometry for determining the atomic masses fairly accurately. But, in the nineteenth century, scientists could determine mass of one atom relative to another by experimental means.
Hydrogen, being lightest atom was arbitrarily assigned a mass of 1 (without any units) and other elements were assigned masses relative to it. However, the present system of atomic masses is based on carbon – 12 as the standard and has been agreed upon in 1961. Here, Carbon – 12 is one of the isotopes of carbon and can be represented as 12C. In this system, 12C is assigned a mass of exactly 12 atomic mass unit (amu) and masses of all other atoms are given relative to this standard. One atomic mass unit is defined as a mass exactly equal to one-twelfth the mass of one carbon – 12 atom.
And 1 amu = 1.66056 x 10-24 g
Mass of an atom of hydrogen = 1.6736 x 10-24 g
Thus, in terms of amu, the mass of hydrogen atom = [1.6736 x 10-24/1.66056 x 10-24]
= 1.0078 amu
= 1.0080 amu
Similarly, the mass of oxygen – 16 (16O) atom would be 15.995 amu.