Physics — "A Comiectlon between the Spectra of Ionized Potas- 

 sium and Argon." (First Coimiiiinicalioii.) Bj Prof. P. Zeeman 

 and H. W. J. Dik. 



(Communicated at the meeting of April 29, 1922). 



According to the conception of Rutherford-Bohr an atom consists 

 of a very small positively charged nucleus, which contains almost 

 the whole mass of the atom, and of a number of electrons revolving- 

 round the nucleus. The number of electrons moving round the 

 nucleus, is equal to the atomic number of the element; hence this 

 also indicates the number of units of charge which an atom that is 

 neutral taken as a whole, possesses in the nucleus. 



It has been made plausible that the electrons are arranged in 

 shells or sheaths with the nucleus as centre. In particular the regular 

 changes which the chemical properties undergo with the inciease 

 of the atomic number, make this probable. Regularly elements occur 

 in the periodic system which easily cede one electron (the alkalis), 

 regular is also the succession of the inert gases. This becomes com- 

 prehensible when it is assumed that a shell can become full, and that 

 then the configuration will be very stable: helium, neon, argon etc. 

 The atoms of lithium, sodium, potassium etc. have only one electron 

 in the outer shell. On this similarity in structure rests also the 

 resemblance which has been observed at an early date in the arc- 

 spectra of the alkalis. The one outer electron can be removed by 

 the electric forces which are active in a spark. Then the atom is 

 ionized, and the electron combination which has remained, can emit 

 the spark spectrum. 



On these general features of the atomic model, in particular on 

 the number of outer electrons which increases at every step in the 

 periodic system, rests a displacement law enunciated by Kossel and 

 SoMMERFELD^), wliicli establishes a connection between the spai-k 

 spectrum of an element and the arc-spectrum of another element 

 which precedes it in the periodic system. If e.g. an electron of the 

 potassium-atom has been driven out, the remaining electron system 

 must present the greatest resemblance with that of argon, and only 



1) KossEL u. SoMMKRFELD, Auswahlpriiicip iind Verscliiebungssalz bei Serien- 

 spectren. Verb deutsch. physik. Gesellscli. 21. Jahrgang 240, 1919. 



5 

 Proceedings Royal Acad. Amsterdam Vol. XXV. 



