History

Main article: History of the periodic table

In Ancient Greece, the influential Greek philosopher Aristotle proposed that there were four main elements: air, fire, earth and water. All of these elements could be reacted to create another one; e.g., earth and fire combined to form lava. However, this theory was dismissed when the real chemical elements started being discovered. Scientists needed an easily accessible, well organized database with which information about the elements could be recorded and accessed. This was to be known as the periodic table.

The original table was created before the discovery of subatomic particles or the formulation of current quantum mechanical theories of atomic structure. If one orders the elements by atomic mass, and then plots certain other properties against atomic mass, one sees an undulation or periodicity to these properties as a function of atomic mass. The first to recognize these regularities was the German chemist Johann Wolfgang Döbereiner who, in 1829, noticed a number of triads of similar elements:

Some triads

Element	Molar mass (g/mol)	Density (g/cm³
chlorine	35.453	0.0032
bromine	79.904	3.1028
iodine	126.90447	4.933
calcium	40.078	1.55
strontium	87.62	2.54
barium	137.327	3.594

In 1829 Döbereiner proposed the Law of Triads: The middle element in the triad had atomic weight that was the average of the other two members. The densities of some triads followed a similar pattern. Soon other scientists found chemical relationships extended beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen, selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as another group.

Dmitri Mendeleev, father of the periodic table

This was followed by the English chemist John Newlands, who noticed in 1865 that when placed in order of increasing atomic weight, elements of similar physical and chemical properties recurred at intervals of eight, which he likened to the octaves of music, though his law of octaves was ridiculed by his contemporaries.^[3] However, while successful for some elements, Newlands' law of octaves failed for two reasons:

0. It was not valid for elements that had atomic masses higher than Ca.
0. When further elements were discovered, such as the noble gases (He, Ne, Ar), they could not be accommodated in his table.

Finally, in 1869 the Russian chemistry professor Dmitri Ivanovich Mendeleev and four months later the German Julius Lothar Meyer independently developed the first periodic table, arranging the elements by mass. However, Mendeleev plotted a few elements out of strict mass sequence in order to make a better match to the properties of their neighbors in the table, corrected mistakes in the values of several atomic masses, and predicted the existence and properties of a few new elements in the empty cells of his table. Mendeleev was later vindicated by the discovery of the electronic structure of the elements in the late 19th and early 20th century.

Earlier attempts to list the elements to show the relationships between them (for example by Newlands) had usually involved putting them in order of atomic mass. Mendeleev's key insight in devising the periodic table was to lay out the elements to illustrate recurring ("periodic") chemical properties (even if this meant some of them were not in mass order), and to leave gaps for "missing" elements. Mendeleev used his table to predict the properties of these "missing elements", and many of them were indeed discovered and fit the predictions well.

With the development of theories of atomic structure (for instance by Henry Moseley) it became apparent that Mendeleev had listed the elements in order of increasing atomic number (i.e., the net amount of positive charge on the atomic nucleus). This sequence is nearly identical to that resulting from ascending atomic mass.

In order to illustrate recurring properties, Mendeleev began new rows in his table so that elements with similar properties fell into the same vertical columns ("groups").

With the development of modern quantum mechanical theories of electron configuration within atoms, it became apparent that each horizontal row ("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. Modern tables have progressively longer periods further down the table, and group the elements into s-, p-, d- and f-blocks to reflect our understanding of their electron configuration.

In the 1940s Glenn T. Seaborg identified the transuranic lanthanides and the actinides, which may be placed within the table, or below (as shown above).

See also

References

• Theodore L. Brown, H. Eugene LeMay, and Bruce E. Bursten (2005). Chemistry:The Central Science (10th edition ed.), Prentice Hall. ISBN 0-13-109686-9. 
• Helmenstine, Marie (2007). "Trends in the Periodic Table". About, Inc.. Retrieved on 2007-01-27.

Further reading

• Mazurs, E.G., "Graphical Representations of the Periodic System During One Hundred Years". University of Alabama Press, Alabama. 1974.
• Bouma, J., "An Application-Oriented Periodic Table of the Elements", J. Chem. Ed., 66, 741 (1989).
• Eric R. Scerri, The Periodic Table: Its Story and Its Significance, Oxford University Press, 2006.
• Imyanitov, N.S., "Mathematical description of dialectic regular trends in the periodic system", Russ. J. Gen. Chem., 69, 509 (1999) [Eng].
• Imyanitov, N.S., "Modification of Various Functions for Description of Periodic Dependences", Russ. J. Coord. Chem., 29, 46 (2003) [Eng].

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