Chemical elements that have been mistakenly "discovered". Further investigation showed that their discovery was either mistaken, that they have been mistaken from an already-known element, or mixture of two elements, or that they indicated a failing in theory where a new element had been assumed rather than some previously unknown behaviour.

Name Symbol Atomic number Actual element Discovery date Discoverer Notes Alabamium Ab 85 Astatine 1931 Fred Allison Alkalinium 87 Francium 1926 Gerald J. F. Druce, Frederick H. Loring Anglohelvetium See Helvetium Arconium 1911 J. W. Nicholson Asterium 1898 Norman Lockyer, Carl David Tolmé Runge, Friedrich Paschen Aurorium 1874 William Huggins Ausenium Ao 93 Neptunium 1934 Enrico Fermi Austrium 31 Gallium 1886 Eduard Linnemann Hypothesised from spectral line observations in the rare-earth mineral orthite. It was later confirmed that these lines were due to the recently discovered gallium. Berzelium Bz 90 Thorium 1901 Charles Baskerville See Carolinium Bohemium 93 Neptunium 1934 Odolen Koblic Carolinium Cn 90 Thorium 1901 Charles Baskerville With Berzelium, erroneously identified as a mixture of three elements, with thorium, during work at the University of North Carolina. Coronium 26 Iron 1869 Charles Augustus Young and William Harkness Hypothesised from a 530.3 nm green emission line in the solar coronal spectrum. Later, around 1902, renamed as Newtonium. In the 1930s recognised as highly ionized iron, rather than a new element. Cyclonium 61 Promethium 1941 H. B. Law, Ohio State University Dakin Dk 85 Astatine 1937 Rajendralal De Cepare Davyum Iridium and rhodium 1877 Serge Kern Decipium Samarium and other rare earths 1878 Marc Delafontaine Thought to have been isolated from the mineral samarskite in 1878. Dianium Niobium and tantalum 1860 Wolfgang Franz von Kobell See Pelopium Didymium Praseodymium and neodymium 1841 Carl Mosander Dorine 85 Astatine 1936 Horia Hulubei, Yvette Cauchois Eka-caesium 87 Francium Eka-rhenium 107 Bohrium Florentium 61 Promethium 1924 Luigi Rolla and Lorenzo Fernandes Ghiorsium Gh 118 Oganesson 1999 Lawrence Berkeley National Laboratory Gnomium Similarities to nickel and iron 1892 Gerhard Krüss and F. W. Schmidt Not discovered, but hypothesised to explain discrepancies within the iron group elements for an early periodic table ordered by mass. The concept of atomic number clarified the situation without requiring the invention of a new element. Helvetium 85 Astatine 1940 Walter Minder Rediscovered in 1942 and named Anglohelvetium Hesperium Es 94 Plutonium 1934 Enrico Fermi Illinium Il 61 Promethium 1926 Smith Hopkins and Len Yntema, University of Illinois at Urbana–Champaign Ilmenium Niobium and tantalum 1847 R. Hermann See Pelopium Lucium 39 Yttrium 1896 Prosper Barrière Masurium Ma 43 Technetium 1925 Walter Noddack, Otto Berg, Ida Noddack Moldavium Mi 87 Francium 1936 Horia Hulubei, Yvette Cauchois Monium 1898 See Victorium Muriaticum 17 Chlorine 1774 Carl Wilhelm Scheele Murium 1785 Claude Louis Berthollet Nebulium Oxygen 1864 William Huggins Neutronium - 0 (Neutronium) 1926 Andreas von Antropoff Newtonium 1902 See Coronium Ninovium Nv 118 Oganesson 1999 Victor Ninov Studies proved inconsistencies in the data shown from Victor Ninov Nipponium Np 75 Rhenium 1908 Masataka Ogawa Pelopium Niobium and tantalum 1846 Heinrich Rose Confused with mixtures of the difficult to distinguish niobium and tantalum, extracted from the mineral tantalite. Polinium 77 Iridium 1828 Gottfried Osann Ptene 76 Osmium 1803 Hippolyte-Victor Collet-Descotils, Antoine François, comte de Fourcroy, Louis Nicolas Vauquelin, Smithson Tennant Protyle 1816 William Prout Russium 87 Francium 1925 D. K. Dobroserdov Sergenium Sg 108 Hassium 1963 Victor Cherdyntsev Sequanium 93 Neptunium 1939 Horia Hulubei Thought to have been present in tantalite ore, actually not present, rather than mis-identified. Unbibium Ubb 122 (Unbibium) 2008 Amnon Marinov Victorium Gadolinium and terbium 1898 William Crookes Virginium Vi (Vm) 87 Francium 1930 Fred Allison Wasium Mixture of rare earths 1862 J. F. Bahr Another element identified in ores from Ytterby.

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