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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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