<P> Given the problems of the proton -- electron model , it was quickly accepted that the atomic nucleus is composed of protons and neutrons , although the precise nature of the neutron was initially unclear . Within months after the discovery of the neutron , Werner Heisenberg and Dmitri Ivanenko had proposed proton -- neutron models for the nucleus . Heisenberg 's landmark papers approached the description of protons and neutrons in the nucleus through quantum mechanics . While Heisenberg 's theory for protons and neutrons in the nucleus was a `` major step toward understanding the nucleus as a quantum mechanical system , '' he still assumed the presence of nuclear electrons . In particular , Heisenberg assumed the neutron was a proton -- electron composite , for which there is no quantum mechanical explanation . Heisenberg had no explanation for how lightweight electrons could be bound within the nucleus . Heisenberg introduced the first theory of nuclear exchange forces that bind the nucleons . He considered protons and neutrons to be different quantum states of the same particle , i.e. , nucleons distinguished by the value of their nuclear isospin quantum numbers . </P>
who came up with the initial concept of protons and neutrons
Werner Heisenberg Dmitri Ivanenko