SOME CONTEMPORARY ADVANCES IN PIIYSICS-X 103 



which (Hsliiiiiiiish the iKTiiiiltcd orhils ma\- l)c, ihey are not those 

 which prexail in the hxclrogen atom. 



(b) Alkali- Metal Atoms. For these there is reason to l)eHe\e that 

 one electron is normalh" located far beyond all the others, and may 

 be supposed to revolve aroimd a "residue" consisting of all the others 

 and the nucleus. At a great distance, the held due to this residue 

 will be ver>- nearh' a central field such as would surround a nucleus 

 of charge +e, — a hydrogen nucleus; for, from a great distance, the 

 nucleus and the electrons of the residue will seem almost to coincide 

 in place. Nearer in. the forces due to the electrons of the residue 

 may be supposed to compound with that due to the nucleus in such 

 a way that a central field, not varying as the in\-erse square, results. 

 Thus rosette orbits may be expected (for this reason I quoted the 

 principle of quantization for such orbits in Section M). An enormous 

 amount of effort has been spent in constructing central fields, such 

 that the rosette orbits obeying the quantum-conditions (2) and (3) 

 have nearly the energy-values which the Stationary States of these 

 atoms are knowm to possess. Always, the emission of a spectrum 

 line is supposed to result from a transition of the outermost or valence 

 electron from one orbit to another, the electrons of the residue being 

 scarcely or not at all affected. Such is the general explanation for the 

 far-reaching and yet imperfect resemblance of the spectra of these 

 metals to that of hydrogen. 



(c) Other Elements. As one passes across the Periodic Table 

 from left to right along any row, the spectra rapidly lose resemblance 

 to the hydrogen spectrum. This is taken to mean that the assump- 

 tion used for alkali metals— the assumption that one electron lies far 

 beyond the others, and executes transitions while the others remain 

 unaffected— departs progressively further from the truth. Evidence 

 exists that simultaneous transitions of two electrons occur, and very 

 likely yet more drastic rearrangements taking place en bloc. 



(d) Building of Atoms by Consecutive "Binding' of Electrons. An 

 atom composed, when complete, of Z electrons arranged about a 

 nucleus bearing the charge -\-Ze, may have been formed originally in 

 Z stages by the consecutive advent of Z electrons, the first annexing 

 itself to the bare nucleus, the second joining itself to the system com- 

 posed of the nucleus and the first, and so on until as many have 

 arrived as the nucleus is able to hold. Each of these stages should 

 be accompanied by the emission of lines belonging to a particular 

 spectrum; the ordinary hydrogen spectrum accompanies the forma- 

 tion of a hydrogen atom by the step-by-step binding of an electron 

 to a nucleus of charge e, the ionized-helium spectrum accompanies 



