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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