Question

Why is the atomic mass of iodine-131 not $126.904g/mol$?

Answer 1

Isotopes are elements with the same number of protons but different numbers of neutrons. This configuration results in 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 $126.904-53$ protons and 74 neutrons. An isotope of this would be iodine-131, with 53 protons and 78 neutrons. This isotope has a mass number of 131.

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 $1.0079-1proton$ and 0 neutrons. Certain conditions can alter the hydrogen atom, resulting in isotopes like "deuterium" - 1 proton and 1 neutron. This is an entirely different atom you're dealing with and thus, it is dubbed as "Hydrogen-2", because you have a mass number of 2 (1 proton + 1 neutron).

$\text{Deuterium is a name for hydrogen-2; Hyderogen is special and is given name for its isotopes}$