In 1932, Werner Heisenberg introduced a new (unnamed) concept to explain binding of the proton and the then newly discovered neutron (symbol n). His model resembled the bonding model for molecule Hydrogen ion, H 2 : a single electron was shared by two protons. Heisenberg's theory had several problems, most notable it incorrectly predicted the exceptionally strong binding energy of He, alpha particles. However, its equal treatment of the proton and neutron gained significance when several experimental studies showed these particles must bind almost equally. In response, Eugene Wigner used Heisenberg's concept in his 1937 paper where he introduced the term "isotopic spin" to indicate how the concept is similar to spin in behavior.