65 these gaps were subsequently filled as chemists

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[65] These gaps were subsequently filled as chemists discovered additional naturally occurring elements. [66] It is often stated that the last naturally occurring element to be discovered was francium (referred to by Mendeleev as eka-caesium ) in 1939. [67] However, plutonium, produced synthetically in 1940, was identified in trace quantities as a naturally occurring primordial element in 1971, [68] and in 2011 it was found that all the elements up to californium can occur naturally as trace amounts in uranium ores by neutron capture and beta decay. [3]
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6/2/13 11:44 AM Periodic table - Wikipedia, the free encyclopedia Page 10 of 17 Short form of periodic table, as originally published by Mendeleev in 1871, updated with all elements discovered to 2012. Glenn T. Seaborg who, in 1945, suggested a new periodic table showing the actinides as belonging to a second f-block series Theodor Benfey's spiral periodic table The popular [69] periodic table layout, also known as the common or standard form (as shown at various other points in this article), is attributable to Horace Groves Deming. In 1923, Deming, an American chemist, published short (Mendeleev style ( - synthesis.com/webbook/35_pt/pt_database.php?PT_id=456)) and medium (18-column ( - synthesis.com/webbook/35_pt/pt_database.php?PT_id=360)) form periodic tables. [70][n 5] Merck and Company prepared a handout form of Deming's 18-column medium table, in 1928, which was widely circulated in American schools. By the 1930s Deming's table was appearing in handbooks and encyclopaedias of chemistry. It was also distributed for many years by the Sargent-Welch Scientific Company. [71][72][73] With the development of modern quantum mechanical theories of electron configurations within atoms, it became apparent that each period (row) in the table corresponded to the filling of a quantum shell of electrons. Larger atoms have more electron sub-shells, so later tables have required progressively longer periods. [74] In 1945, Glenn Seaborg, an American scientist, made the suggestion that the actinide elements, like the lanthanides were filling an f sub-level. Before this time the actinides were thought to be forming a fourth d- block row. Seaborg's colleagues advised him not to publish such a radical suggestion as it would most likely ruin his career. As Seaborg considered he did not then have a career to bring into disrepute, he published anyway. Seaborg's suggestion was found to be correct and he subsequently went on to win the 1951 Nobel prize in chemistry for his work in synthesizing actinide elements. [75][76][n 6] Although minute quantities of some transuranic elements occur naturally, [3] they were all first discovered in laboratories. Their production has expanded the periodic table significantly, the first of these being neptunium, synthesized in 1939.
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