Why is the atomic mass of iodine 131 not 126.904 g/mol?
Hint: Every atom is composed of a nucleus and this nucleus is made up of protons and neutrons where protons is positively charged and neutrons are neutral in charge and electrons which are negatively charged are revolving around the nucleus.
Complete answer: The mass number reports the mass of the atoms present in the nucleus in atomic mass units represented by the term amu. Now in the given case we are considering the atomic mass of iodine is 131 not 126.904 this can be explained on the concept of isotopes where isotopes can be defined as those compounds which have the same chemical element but different atomic mass numbers. Atomic mass number is given by the total number of protons and neutrons. These have the same number of protons but different numbers of neutrons and the number next to an isotope is mass number. For a typical iodine compound atomic mass should be 126.904 g/mol which contains 53 protons and 74 neutrons. On the other hand an isotope of iodine-131 which has 53 protons and 78 neutrons have mass number 131. Hence we can say that 126.904 is the average atomic mass of Iodine and Iodine-131 is an isotope of Iodine.
Note: Atomic number and mass number are always whole numbers because these are calculated by counting whole objects i.e. protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom corresponds to the total number of subatomic particles present in the atom.