Most of the elements occur in nature as a mixture of two or more isotopes. To determine atomic mass of such an element, it is necessary to know not only the masses of the individual isotpes but also their atom percents (isotopic abudance) in nature.
The mass spectrum gives us all the information needed to calculate the atomic weight; the masses of isotope and their relative numbers or isotopic abudances also called fractional abundances.
The isotopic or fractional abundance of an isotope is the fraction of the total number of atoms that is composed of a perticular isotope.
For chlorine which has two stable isotopes, Cl-35 and Cl-37. The atomic masses and relative abundances of these isotopes as determined by mass spectroscopy is as follows
atomic mass abudance
Cl-35 34.97 amu 75.53%
Cl-37 36.97 amu 24.47%
We interpret this to mean that in elemental chlorine, 75% of the atoms have a mass of 34.97 amu and the remaining atoms 24.47% of the total, have a mass of 36.97 amu. With this information we can readily calculate the atomic mass of chlorine using general equation.
Atomic mass Y = ( atomic mass Y1) x %Y1 (atomic mass Y2) x %Y2
----------- + ----------- + ..........
where Y, Y2.... are the isotopes of element Y.
atomic mass of Cl= 34.97 amu x 75.53 36.97 amu x 24.47
----------- + .........
= 35.46 amu
Example; What is the atomic mass of chromium?
mass number isotopic mass(amu) isotopic abundance(amu)
50 49.94 0.0435
52 51.94 0.837
53 52.94 0.0950
54 53.93 0.023
atomic mass of Cl = 49.94 x 0.0435 + 51.94 x 0.837+52.94 x 0.0950+ 53.93 x 0.023
= 51.99 amu