Elecron spin
Enrico Fermi ( 1901-1954) and Paul Dirac (1902-1984) formulate the theories of the behaviour of fermions, these corpuscles which could have only two states of rotation or spin with respect to an axis ; one called left handed, the other right handed. Electrons can not be in the same 'quantic state' according to the Fermi-Dirac theory. They organize themselves to form voluminous clouds around atomic nuclei.