Missing Elements in Periodic Table.
By 1920, almost all the elements in Mendeleev's periodic table had been Found There were only six gaps, i.e., missing elements. All these were found during the next 25 years. We give here a brief summary of how these six elements were found. Three of the women physicists for whom there are citations in the CWP site played an important role in these discoveries. Some of the information is summarized in the table below:
DISCOVERY OF CHEMICAL ELEMENTS AFTER 1920 ELEMENT ATOMIC NUMBER OCCURRENCE YEAR DISC. DISCOVERER(S)
Hafnium 72 Naturally stable 1923 Coster
Hevesy Rhenium 75 Naturally stable 1925 Ida W. Noddack (CWP) Noddack citation
Technicium 43 Nuclear Reaction 1937 Perrier & Segre
Francium 87 Natural Radioactivity 1939 M. Perey (CWP) Perey citation
Astatine 211 85 Nuclear Reaction 1940 Corson, McKenzie
Segre Astatine 215,218 85 Natural Radioactivity 1941 B. Karlik (CWP) Karlik citation
Promethium 61 Fission Product 1945 Marinski,Glendenin&,Coryell
Before 1930, all the elements discovered were either naturally stable, or belonged to one of the naturally radioactive chains. (The latter all have atomic numbers between 80 and 92.) After discovering Rhenium by using X-ray spectroscopy, The Noddacks Noddack citationalso claimed to have found the element with Z = 43, using the same methods. In fact, these two elements are expected to be chemically similar. However, the element Z = 43, now known as Technicium, is not stable. Its longest lived isotope with A = 99, decays by beta decay with a half life of 10^6 years. It cannot be part of one of the known naturally radioactive chains. Also, at the time (1925), nuclear reactions were just beginning to be used. Induced radioactive substances were first made in the 1930's by Irene and Frederick Joliot-Curie, and fission was not discovered until 1938. Therefore Technicium could not have been discovered in the 1920's. A similar situation occurred with Promethium, when, in 1926, Hopkins et al believed that they had discovered it, using X-ray lines. Again, this substance does not have any stable isotopes, and it is too light to occur in the natural radioactive chain.