By 1925, the problematic chlorine was found to be composed of the isotopes 35Cl and 37Cl, in proportions such that the average weight of natural chlorine was about 35.45 times that of hydrogen. For all elements, each individual isotope of mass number "A" was eventually found to have a mass very close to "A" times the mass of a hydrogen atom, with an error always less than 1%. This is a near miss to Prout's law being correct. Nevertheless, the rule was not found to predict isotope masses better than this for all isotopes, due mostly to mass defects resulting from release of binding energy in atomic nuclei when they are formed.