For many elements with atomic number Z small enough to occupy only the first three nuclear shells, that is up to that of calcium (Z = 20), there exists a stable isotope with N / Z ratio of one. The exceptions are beryllium (N / Z = 1.25) and every element with odd atomic number between 9 and 19 inclusive (though in those cases N = Z + 1 always allows for stability). Hydrogen-1 (N / Z ratio = 0) and helium-3 (N / Z ratio = 0.5) are the only stable isotopes with neutron–proton ratio under one. Uranium-238 has the highest N / Z ratio of any primordial nuclide at 1.587, while mercury-204 has the highest N / Z ratio of any known stable isotope at 1.55. Radioactive decay generally proceeds so as to change the N / Z ratio to increase stability. If the N / Z ratio is greater than 1, alpha decay increases the N / Z ratio, and hence provides a common pathway towards stability for decays involving large nuclei with too few neutrons. Positron emission and electron capture also increase the ratio, while beta decay decreases the ratio.