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 … )
(As of 11/09/2018, 7h10 : )
There is an observation to be made, about the quantity defined in the link above, which is called a ‘mole’, and which is often used in Physics and Chemistry, to compute the amounts of reagents versus reaction products needed, to carry out some experiment or process.
Currently, the molar quantity is defined as the number of atoms, of the isotope Carbon-12, that would be needed, to achieve a mass of exactly 12 grams. This number is also referred to as the Avogadro Constant, and is approximately 6.022 · 1023 .
This is not a magic number. The unit could be redefined, so that the required quantity of Hydrogen-1 atoms has a mass of exactly 1 gram. But if we did that, then this would actually change the resulting number slightly, if that number was written with greater precision to begin with, than the precision with which we know the mass of Hydrogen-1 atoms.
The point of my posting was merely to explain, that if a molar quantity of Carbon-12 weighs 12 grams, then a molar quantity of Hydrogen-1 atoms will not weigh exactly 1 gram, and vice-versa.
(Update 11/10/2018, 18h00 : )
Scientists refer to this observation as “Mass Defect“. However, certain popular definitions of that term run in to erroneous interpretations. The main source of error would be the fact, that even as nuclei approach each other, the energy potential they overcome increases with the size of the fused nucleus, but only in certain cases the reaction is exothermic, to produce a fused nucleus. This is why our Sun does not collapse, as the net result of Fusion at its core releases heat, which in turn allows its life to continue.
Well this also means that the potential energy of certain nuclei is lower, than that of the original particles. Regardless of how the energy was dissipated, there can be net energy released, and this must leave the system with nuclei that are lighter than the original particles were.
However, past a certain atomic number, the electrostatic repulsion between protons becomes stronger than the strong nuclear force which holds the nucleus together, so that further Fusion, leads to a net result of being endothermic.
Endothermic reactions are ultimately possible, because either the molecule or nucleus which forms, has the ability to store energy. In the case of nuclear chemistry, all the atomic numbers greater than a certain number, experience the effect of the electrostatic repulsion being stronger than the strong nuclear force. But, some of those nuclei simply exhibit exceptional stability.
This is why harnessed nuclear Fission can derive energy from Uranium, Thorium, or Plutonium fuels. And in such cases, the mass of the resulting nucleus would increase over those of the original nuclei added together, when further Fusion takes place, let’s say inside a supergiant star about to go supernova…
To the best of my understanding, while electrostatic repulsion always favors Fission, the strong nuclear force always favors Fusion.