increases, though undiscovered nuclides with certain "magic" numbers of protons and neutrons may have relatively longer half-lives and comprise an island of stability. A hypothetical element composed only of neutrons has also been proposed and would have atomic number 0. See also Atomic theory Chemical element Effective atomic number (disambiguation) Even and odd atomic nuclei Exotic atom History of the periodic table List of elements by atomic number Mass number Neutron number Neutron–proton ratio Prout's hypothesis References Chemical properties Nuclear physics Atoms Dimensionless numbers of chemistry Numbers