Atomic Weights

Most elements can be found on earth (with the exception of those elements that too unstable and thus must be synthesized in the laboratory). Since all elements have isotopes then we must consider how much of one isotope of an element exists versus another isotope of the same element. These are called the "natural" abundances on earth.

Natural Abundances:

Suppose we go to a cave and mine element "X". After careful analysis we find that in our sample of element X there exists three isotopes: Xa, Xb and Xc. Moreover, we find that out of every 100 atoms the various isotopes are distributed as follows:

 For Every 100 atoms of X


  No. of atoms
 Xa  30
 Xb  60
Xc  10

Then we say that the natural percent abundance of Xa is 30 %, of Xb is 60 % and of Xc is 10 %.

Next, we can inquire what the mass of element X is? Since each isotope has a different mass (because each isotope has a different number of neutrons) the simplest answer is to give the "average" mass of element X - the atomic weight. After more analysis the mass of each isotope is determined to be the following:

Isotope Masses of X


 Mass (amu)
 Xa  54
 Xb  56
Xc  59

Then the average mass (atomic weight) is given by:

The atomic weight of each element is included along with the element symbol in the periodic table. It is important to note that no one atom has a mass equal to that of the atomic weight. Remember: the atomic weight represents that average mass of the atoms.

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C101 Class Notes
Prof. N. De Leon