Proton-neutron nucleus

Heisenberg suggested that these recently discovered neutrons must live inside the nucleus. This neatly explained anomalies in nuclear spins and the existence of isotopes. A hydrogen-1 atom contained just a proton and an electron, while a hydrogen-2 atom also contained a neutron. However why a nucleus packed with positively charged protons could be stable remained a mystery.
 Werner Karl Heisenberg Germany 1932 Proton-neutron nucleus

Heisenberg suggested that these recently discovered neutrons must live inside the nucleus. This neatly explained anomalies in nuclear spins and the existence of isotopes. A hydrogen-1 atom contained just a proton and an electron, while a hydrogen-2 atom also contained a neutron. However why a nucleus packed with positively charged protons could be stable remained a mystery.