Atomic mass (or ATOMIC WEIGHT, peso atomico) is the average of the masses of the possible isotopes, weighed on NATURAL ABUNDANCES H
1H
= 99.985% 1.007825 u
2H
(D) = 0.015% 2.04102 u
Average atomic mass of H H = 0.99985 x 1.007825 + 0.00015 x 2.014102 amu = 1.00797 amu