The amount that the ratio of atomic masses to mass number deviates from 1 is as follows: the deviation starts positive at hydrogen-1, becomes negative until a minimum is reached at iron-56, iron-58 andnickel-62, then increases to positive values in the heavy isotopes, with increasing atomic number. This corresponds to the fact that nuclear fission in an element heavier than zirconium produces energy, and fission in any element lighter than niobium requires energy. On the other hand, nuclear fusion reactions: fusion of two atoms of an element lighter than scandium produces energy, whereas fusion in elements heavier than calcium requires energy.
The difference mass number minus atomic mass is not maximized at iron-56 (where it is around 0.06 atomic mass units), but at heavier elements where it reaches about 0.10 atomic mass units.
Here are some values of the ratio of atomic mass to mass number:
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