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

1 Answer
Jan 29, 2017


#126.904# is the (average) atomic mass of Iodine, and Idodine-131 is an isotope of Iodine.


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 #126.904# - 53 protons and 74 neutrons. An isotope of this would be Iodine-131, with 53 protons and 78 neutrons. This isotope has an 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# - 1 proton 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 an mass number of 2 (1 proton + 1 neutron).

Deuterium is a name for Hydrogen-2; Hydrogen is special and is given a name for its isotopes

Hope this helps :)