When we have certain reference-books at our disposal, one of the things we can do, is to look up what the atomic mass is, of individual isotopes. And, the CRC Handbook of Physics and Chemistry, 61st Edition, 1980-1981, already had such information available. The fact that it was so long ago as 1980, did not prevent Mankind from designing H-Bombs etc.. And this is a short excerpt from that handbook:
n1 1.008665 H1 1.007825 H2 2.0140 H3 3.01605 He3 3.01603 He4 4.00260 Li5 5.0125 Li6 6.01512 Li7 7.01600 (...) C12 12 (...) Fe56 55.9349 (...) U238 238.0508
There’s an important fact to observe about this. The atomic masses listed above, do not result because each naturally-occurring element, occurs as a mixture of more than one isotope. They do, but this does not give rise to the numbers listed above. What we find instead is that for each isotope, except for Carbon-12, the atomic mass is slightly different, from that isotope’s mass-number. This is not an error.
Well, when a fission reactor produces heat, or, when an H-Bomb explodes, it’s from these discrepancies in the atomic mass, that either device realizes energy, according to the famous equation, E=mc2 . So, these discrepancies in mass, are converted into energy, and it’s only when energy is output on such a large scale, that an associated difference in mass starts to become measurable.
What should also be noticed is that for the lightest elements as shown above, the atomic masses are generally slightly greater than the mass-numbers, which is consistent with the fact that Fusion releases energy. For elements much heavier than Iron, such as Uranium, the atomic masses are also generally greater than the mass-numbers, which is consistent with the fact that Fission releases energy. But near the occurrence of Iron in the isotope table, the atomic masses are generally slightly less than the mass-numbers. And this latter fact is consistent with the fact that when Carbon fuses into heavier elements, again, some amount of energy is released. The potential energy is at a minimum, when a given quantity of Iron is being measured. And possible differences, in ? one Iodine nucleus giving rise to two Iron nuclei ? , must also be taken into consideration, when computing the energy balance.
This last detail means, that one Iodine atom may have even-lower potential energy, than one Iron atom. But the depression of one Iodine atom’s mass, below its mass-number, will not be twice as great, as that for Iron.
(Update 11/07/2018, 8h35 : )
When the mass of an atom or a molecule is being stated in Physics or Chemistry, The units used are usually gram /mole.
(Updated 11/10/2018, 18h00 … )