Why are atomic masses of most of the elements fractional?
Most elements occur as a mixture of isotopes of different masses. The fractional atomic masses arise because of this mixture.
Avg. mass = total mass of all atoms/number of atoms.
Before we calculate the average mass of atoms, let's use an analogy.
Now let's use this technique to calculate the average mass of a mixture of isotupes.
If 80.0% of an element has an atomic mass of 42.0 u and 20.0% has an atomic mass of 43.0 u, what is the average mass of the mixture?
Let's assume we have 1000 atoms. Then 800 will have mass 42.0 u, and 200 will have mass 43.0 u.
The average mass is the mass of each isotope multiplied by its percentage. Thus,
The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.