How do you calculate average atomic masses of elements?

Answer 1

Many elements consist of more than one naturally occurring isotope. The average atomic mass of an element is the "weighted" average. This means that more "weight" or importance is given to the isotope which is in greater natural abundance.

We know the percentages of each isotope. To do the calculation we will express each percentage as its corresponding decimal fraction.

Average atomic mass = (fraction1 x atomic mass1) + (fraction2 x atomic mass2) + (fraction3 x atomic mass3) ...

Cstspeedy has given you the gist of the calculation, and has mentioned that the numbers used will only produce an approximation of the average atomic mass. What should be used are more precise measurements of the percent abundances and the actual masses of the isotopes.

This is the important part. The mass numbers are NOT the actual masses of the isotopes. They are close, but not the same. The actual isotopic masses are known very precisely.

For instance, Cl-37 has a mass of 36.965902591 amu. We don't need to use all those digits, but they represent the actual mass of the isotope.

So for the chlorine example, where there are only two naturally occurring isotopes, you would use the following. 75.58 percent of chlorine atoms are chlorine-35 and 24.22 percent of chlorine atoms are chlorine-37.

The average atomic mass = 0.7578 x 34.9689 + 0.2422 x 36.9659 = 35.453 amu