How is atomic mass determined?
Here is an example:
Chlorine exists in the form of two principle isotopes. One has 17 protons and 18 neutrons, and a mass of 34.969 u. It makes up 75.78% of all chlorine atoms. The other isotope has 17 protons and 20 neutrons, and accounts for the remaining 24.22% of atoms. The mass of these atoms is 36.966 u.
(u is the atomic mass unit equal to
Now, find the average mass of these atoms:
Add the two above values:
So, the average mass of a chlorine atom is 35.45 u. This is the atomic mass.
An atomic mass very close to a whole number usually means the element is composed of only one major isotope (with very small portions of lighter of heavier isotopes.) Chlorine has two major isotopes, and this leads to an atomic mass not close to a whole number.