XII.—A Static Model for Helium

Abstract

Helium, with atomic number 2, stands second only to hydrogen in interest for the theoretical physicist. The atom would have to consist of a doubly charged positive nucleus and two electrons. “Here,” as Sommerfeld puts it, “we stumble over the three-body problem.” In Bohr's original model of the neutral helium atom the two electrons rotate about the nucleus at opposite ends of a diameter of a circle. The work necessary to detach both electrons, or the second order ionisation potential (if it is expressed in equivalent volts), comes out too great, 82·9 volts instead of 78·6 volts. Later models conceived by Landé and by Bohr clear up some of the difficulties of explaining the separate sets of series described as the parhelium (singlet) and the orthohelium (doublet) series, but according to the calculations of Kramers, the ionisation potential of the crossed orbit model comes out 3·8 volts less than the value experimentally determined. Summing up the present position in his Presidential Address to Section A, at the Liverpool meeting of the British Association, M'Lennan says: “Although real progress has been made, it cannot be said that finality has been reached in the determination of the form of a completely satisfactory model of the atom of so simple an element as helium.”