Periodic table

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The periodic table of the chemical elements is a tabular display of the known chemical elements. The elements are arranged by electron structure so that many chemical properties vary regularly across the table. Each element is listed by its atomic number and chemical symbol.

The standard table provides the necessary basics. There are also other methods for displaying the chemical elements for more details or different perspectives.

Groups
A group is a vertical column in the periodic table of the elements. There are 18 groups in the standard periodic table. Elements in a group have similar configurations of their valence shell electrons, which gives them similar properties.

Group numbers
There are three systems of group numbers; one using Arabic numerals and the other two using Roman numerals. The Roman numeral names are the original traditional names of the groups; the Arabic numeral names are a newer naming scheme recommended by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC scheme was developed to replace both older Roman numeral systems as they confusingly used the same names to mean different things.

Standard periodic table
State at standard temperature and pressure
 * those in red are gases
 * those in green are liquids
 * those in black are solid

Natural occurrence
 * those with solid borders have isotopes that are older than the Earth (Primordial elements)
 * those with dashed borders naturally arise from decay of other chemical elements
 * those with dotted borders do not occur naturally (Synthetic elements)
 * those without borders have not been discovered yet

Other methods for displaying the chemical elements

 * The standard table (same as above) provides the basics.
 * A vertical table for improved readablity in web browsers
 * The big table provides the basics plus full element names.
 * The huge table provides the basics plus full element names and atomic masses.
 * A table with an inline F-block inserts the Lanthanides and Actinides back into the table
 * The elements to 218 suggests the places so-far undiscovered elements would be
 * Antimatter table
 * Electron Configurations
 * Metals and Non Metals
 * Periodic table filled by blocks
 * Table in Chinese
 * List of elements by name
 * List of elements by symbol
 * List of elements by atomic number
 * List of elements by boiling point
 * List of elements by melting point
 * List of elements by density
 * List of elements by atomic mass

And here is the periodic table for magnetic resonance.

Explanation of the structure of the periodic table
The number of electron shells an atom has determines to which period it belongs. Each shell is divided into different subshells, which as atomic number increases are filled in roughly this order:

1s 2s          2p 3s          3p 4s       3d 4p 5s       4d 5p 6s    4f 5d 6p 7s    5f 6d 7p 8s 5g 6f 7d 8p ...

Hence the structure of the table. Since the outermost electrons determine chemical properties, those tend to be similar within groups. Elements adjacent to one another within a group have similar physical properties, despite their significant differences in mass. Elements adjacent to one another within a period have similar mass but different properties.

For example, very near to nitrogen (N) in the second period of the chart are carbon (C) and oxygen (O). Despite their similarities in mass (they differ by only a few atomic mass units), they have extremely different properties, as can be seen by looking at their allotropes: diatomic oxygen is a gas that supports burning, diatomic nitrogen is a gas that does not support burning, and carbon is a solid which can be burnt (yes, diamonds can be burnt!).

In contrast, very near to chlorine (Cl) in the next-to-last group in the chart (the halogens) are fluorine (F) and bromine (Br). Despite their dramatic differences in mass within the group, their allotropes have very similar properties: They are all highly corrosive (meaning they combine readily with metals to form metal halide salts); chlorine and fluorine are gases, while bromine is a very low-boiling liquid; chlorine and bromine at least are highly colored.

History
Main article: History of the periodic table

The original table was created without a knowledge of the inner structure of atoms: 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:

This was followed by the English chemist John Alexander Reina Newlands, who in 1865 noticed that the elements of similar type recurred at intervals of eight, which he likened to the octaves of music, though his law of octaves was ridiculed by his contemporaries. Finally, in 1869, the German Lothar Meyer and the Russian chemist Dmitri Ivanovich Mendeleev almost simultaneously 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 neighbours 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.

Further resources

 * 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).