Why is the atomic mass of iodine-131 not #"126.904 g/mol"#?
You are confusing isotopes with the atomic mass.
Isotopes are elements with the same number of protons but different number of neutrons. This configuration results in a different stability of the same element being dealt with. The number next to an isotope is the mass number.
For example, a typical iodine atom has an atomic mass of
Remember that atomic mass is an average of all possible masses of a certain element.
Another example of an isotope is Hydrogen-2. A typical hydrogen atom has an average atomic mass of
Deuterium is a name for Hydrogen-2; Hydrogen is special and is given a name for its isotopes
Hope this helps :)