MISSING ELEMENTS IN THE PERIODIC TABLE

by

S. A. Moszkowski

By 1920, almost all the elements in Mendeleev's periodic table had been found. There were only six gaps, i.e., missing elements. At the time, the periodic table ended at Uranium (atomic number 92), which, until 1940, was the heaviest of the known elements. The table below shows the periodic table up to Z = 92, highlighting those elements discovered after 1920.

MISSING  ELEMENTS  AS  OF  1920

  H 1              
He 2 Li 3 Be 4 B 5 C 6 N 7 O 8 F 9  
Ne 10 Na 11 Mg 12 Al 13 Si 14 P 15 S 16 Cl 17  
Ar 18 K 19 
     Cu29		 Ca20 
     Zn30		 Sc21 
    Ge31		 Ti22 
     Ge32		 V23 
     As33		 Cr24 
     Se34		 Mn25 
     Br35		 Fe,Co,Ni
Kr 36 Rb37 
      Ag47		 Sr38 
     Cd48		 Y39 
     In49		 Zr40 
      Sn50 		 Nb41 
      Sb51		 Mo42 
      Te52		 Tc43 
     I53		 Ru,Rh,Pd
Xe 54 Cs55 
      Au79		 Ba56 
      Hg80		 Pm61 
      Tl81		 Hf72 
     Pb82 		 Ta73 
       Bi83		 W74 
      Po84		 Re75 
    At85		 Os,Ir,Pt
Rn 86 Fr87 Ra 88 Ac 89 Th 90 Pa 91 U 92    

All the six missing elements were found during the next 25 years, three of them by women physicists, Ida Noddack, Marguerite Perey, and Berta Karlik . Some information regarding these discoveries is summarized in the table below:
 
DISCOVERY OF CHEMICAL ELEMENTS AFTER 1920 
Element Atomic Number Occurrence Year Discovered Discoverer(s)
Hafnium
72
 Naturally stable
1923
 Coster & Hevesy
Rhenium
75
 Naturally stable
1925
 Noddack citation
Technicium
43
 Nuclear Reaction
1937
 Perrier & Segre
Francium
87
 Natural Radioactivity
1939
 Perey citation
Astatine 211
85
 Nuclear Reaction
1940
 Corson & McKenzie
  Astatine 215,218
85
 Natural Radioactivity
1941
 Karlik citation
Promethium
61
 Fission Product
1945
 Marinski, Glendenin & Coryell

Induced radioactive substances were made in the 1930's by Irene and Frederic Joliot-Curie .

Fission was discovered in 1938 by Otto Hahn, Lise Meitner , and Arnold Strassmann.


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