Although precursors exist, the current table is generally credited to Dmitri Mendeleev, who developed it in 1869 to illustrate periodic trends in the properties of the then-known elements; the layout has been refined and extended as new elements have been discovered and new theoretical models developed to explain chemical behavior. Mendeleev's presentation also predicted some properties of then-unknown elements expected to fill gaps in his arrangement; these predictions were proved correct when those elements were discovered and found to have properties close to the predictions.
All elements from atomic numbers 1 (hydrogen) to 118 (ununoctium) have been isolated. Of these, all up to californium exist naturally; the rest have only been artificially synthesised in laboratories, along with numerous synthetic radionuclides of naturally occurring elements. Production of elements beyond ununoctium is being pursued, with the question of how the periodic table may need to be modified to accommodate these elements being a matter of ongoing debate.
History
Main article: History of the periodic table
Dmitri Mendeleev
Previous attempts at systemization
In 1789, Antoine Lavoisier published a list of 33 chemical elements. Although Lavoisier grouped the elements into gases, metals, nonmetals, and earths,[1] chemists spent the following century searching for a more precise classification scheme. In 1829, Johann Wolfgang Döbereiner observed that many of the elements could be grouped into triads (groups of three) based on their chemical properties. Lithium, sodium, and potassium, for example, were grouped together as soft, reactive metals. Döbereiner also observed that, when arranged by atomic weight, the second member of each triad was roughly the average of the first and the third.[2] This became known as the Law of Triads.[3] German chemist Leopold Gmelin worked with this system, and by 1843 he had identified ten triads, three groups of four, and one group of five. Jean-Baptiste Dumas published work in 1857 describing relationships between various groups of metals. Although various chemists were able to identify relationships between small groups of elements, they had yet to build one scheme that encompassed them all.[2]
German chemist August Kekulé had observed in 1858 that carbon has a tendency to bond with other elements in a ratio of one to four. Methane, for example, has one carbon atom and four hydrogen atoms. This concept eventually became known as valency. In 1864, fellow German chemist Julius Lothar Meyer published a table of the 49 known elements arranged by valency. The table revealed that elements with similar properties often shared the same valency.[4]
English chemist John Newlands produced a series of papers in 1864 and 1865 that described his own classification of the elements: He noted that when listed in order of increasing atomic weight, similar physical and chemical properties recurred at intervals of eight, which he likened to the octaves of music.[5][6] This Law of Octaves, however, was ridiculed by his contemporaries, and the Chemical Society refused to publish his work.[7] Nonetheless, Newlands was able to draft an atomic table and use it to predict the existence of missing elements, such as germanium. The Chemical Society only acknowledged the significance of his discoveries some five years after they credited Mendeleev.[citation needed]
Mendeleev's table
Mendeleev's 1869 periodic table; note that his arrangement presents the periods vertically, and the groups horizontally
Russian chemistry professor Dmitri Ivanovich Mendeleev and German chemist Julius Lothar Meyer independently published their periodic tables in 1869 and 1870, respectively.[8] They both constructed their tables in a similar manner: By listing the elements in a row or column in order of atomic weight and starting a new row or column when the characteristics of the elements began to repeat.[9] The success of Mendeleev's table came from two decisions he made: The first was to leave gaps in the table when it seemed that the corresponding element had not yet been discovered.[10] Mendeleev was not the first chemist to do so, but he was the first to be recognized as using the trends in his periodic table to predict the properties of those missing elements, such as gallium and germanium.[11] The second decision was to occasionally ignore the order suggested by the atomic weights and switch adjacent elements, such as cobalt and nickel, to better classify them into chemical families. With the development of theories of atomic structure, it became apparent that Mendeleev had listed the elements in order of increasing atomic number.[12]
Further development
In the years following publication of Mendeleev's periodic table, the gaps he identified were filled as chemists discovered additional naturally occurring elements.[13] It is often stated that the last naturally occurring element to be discovered was francium (referred to by Mendeleev as eka-caesium) in 1939.[14] However, plutonium, produced synthetically in 1940, was identified in trace quantities as a naturally occurring primordial element in 1971.[15]
With the development of modern quantum mechanical theories of electron configurations within atoms, it became apparent that each row (or period) in the table corresponded to the filling of a quantum shell of electrons. In Mendeleev's original table, each period was the same length. However, because larger atoms have more electron sub-shells, modern tables have progressively longer periods further down the table.[16]
The production of various transuranic elements has expanded the periodic table significantly, the first of these being neptunium, synthesized in 1939.[17] Because many of the transuranic elements are highly unstable and decay quickly, they are challenging to detect and characterize when produced, and there have been controversies concerning the acceptance of competing discovery claims for some elements, requiring independent review to determine which party has priority, and hence naming rights. The most recently named element is copernicium (number 112), named on 19 February 2010;[18] the most recently accepted discoveries are ununquadium (114) and ununhexium (116), both accepted on 1 June 2011.[19]. In 2010, a joint Russia–US collaboration at Dubna, Moscow Oblast, Russia, claimed to have synthesized six atoms of ununseptium, making it the most recently claimed discovery.
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