The periodic table is a tabular display of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and chemical properties. Elements are presented in increasing atomic number. The main body of the table is a 18 × 7 grid, and elements with the same number ofvalence electrons are kept together in groups, such as the halogens and the noble gases. There are four distinct rectangular areas or blocks. The f-block is usually not included in the main table, but rather is floated below, as an inline f-block would often make the table impractically wide. Using periodic trends, the periodic table can help predict the properties of various elements and the relations between properties. As a result, it provides a useful framework for analyzing chemical behavior, and is widely used in chemistry and other sciences.
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; most of 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 synthesized. Of these, all up to and including californium exist naturally; the rest have only been artificially synthesised in laboratories. Production of elements beyond ununoctium is being pursued, with the question of how the periodic table may need to be modified to an extended form to accommodate these elements being a matter of ongoing debate. Numerous syntheticradionuclides of naturally occurring elements have also been produced in laboratories.
History
First systematization attempts
In 1789, Antoine Lavoisier published a list of 33 chemical elements, grouping them into gases, metals, nonmetals, and earths;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 based on their chemical properties. Lithium, sodium, and potassium, for example, were grouped together in a triad 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;this became known as the Law of Triads.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.
In 1858, German chemist August Kekulé observed that carbon often has four other atoms bonded to it. Methane, for example, has one carbon atom and four hydrogen atoms. This concept eventually became known as valency; different elements bond with different numbers of atoms. 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.
English chemist John Newlands produced a series of papers in 1864 and 1865 noting that when the elements were listed in order of increasing atomic weight, similar physical and chemical properties recurred at intervals of eight; he likened such periodicity to the octaves of music.This Law of Octaves, however, was ridiculed by Newlands' contemporaries, and the Chemical Society refused to publish his work.Newlands was nonetheless able to draft a table of the elements and used it to predict the existence of missing elements, such as germanium. The Chemical Society only acknowledged the significance of his discoveries five years after they credited Mendeleev.
Mendeleev's table
Russian chemistry professor Dmitri Mendeleev and German chemist Julius Lothar Meyer independently published their periodic tables in 1869 and 1870, respectively.They both constructed their tables by listing the elements in rows or columns in order of atomic weight and starting a new row or column when the characteristics of the elements began to repeat.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. 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.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 or nuclear charge.
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. It is often stated that the last naturally occurring element to be discovered was francium (referred to by Mendeleev as eka-caesium) in 1939 However, plutonium, produced synthetically in 1940, was identified in trace quantities as a naturally occurring primordial element in 1971,and in 2011 it was found that all the elements up to californium can occur naturally in trace amounts in uranium ores by neutron capture and beta decay.
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 modern tables have progressively longer periods further down the table.
Although minute quantities of some transuranic elements occur naturally,they were all first discovered in laboratories. Their production has expanded the periodic table significantly, the first of these being neptunium, synthesized in 1939.Because many of the transuranic elements are highly unstable and decay quickly, they are challenging to detect and characterize when produced. 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 accepted and named elements are flerovium (element 114) and livermorium (element 116), both named on 31 May 2012.In 2010, a joint Russia–US collaboration at Dubna, Moscow Oblast, Russia, claimed to have synthesized six atoms of ununseptium (element 117), making it the most recently claimed discovery
