[a] The assumption as discussed by Rutherford was of a nucleus consisting of Z + N = A protons plus N electrons somehow trapped within thereby reducing the positive charge to +Z as observed and vaguely explaining beta decay radioactivity.
Such a nuclear constitution was known to be inconsistent with dynamics either classical or early quantum but seemed inevitable until the neutron hypothesis by Rutherford and discovery by English physicist James Chadwick.
The discrepancy between Prout's hypothesis and the known variation of some atomic weights to values far from integral multiples of hydrogen, was explained between 1913 and 1932 by the discovery of isotopes and the neutron.
According to the whole number rule of Francis Aston, Prout's hypothesis is correct for atomic masses of individual isotopes, with an error of at most 1%.
The missing 0.9% of mass represents the energy lost when the nucleus of iron was made from hydrogen inside a star (see stellar nucleosynthesis).