Periodic table : Periodic Table 

In Chemistry, data about the physical properties of the elements can presented in several different ways. The periodic table of the chemical elements is a display of the known chemical elements, arranged by electron structure so that many chemical properties vary regularly across the 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 Johann Wolfgang Döberreiner who noticed a number of triads of similar elements:

Some Triads
Element	Atomic Mass	Density
Cl	35.5	1.56 g/L
Br	79.9	3.12 g/L
I	126.9	4.95 g/L
Ca	40.1	1.55 g/cm3
Sr	87.6	2.6 g/cm3
Ba	137	3.5 g/cm3

This was followed by the Englishman John Alexander Reina Newlands, who 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 the German Lothar Meyer and the Russian chemist Dmitry Ivanovich Mendeleev almost simultaneously developed the first periodic table, arranging the elements by mass (though 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 - this was later vindicated by the discovery of the electronic structure of the elements in the late 19th and early 20th century.)

Lists of the elements by name, by symbol, and by atomic number are available. The following figure shows the currently known periodic table of the elements. Each element is listed by its atomic number and chemical symbol. Elements in the same column ("group") are chemically similar.

Chemical Series of the Periodic Table

Alkali metals	Alkaline earths	Lanthanide	Actinides	Transition metals
Poor metals[?]	Metalloids	Nonmetals	Halogens	Noble gases

Colour coding for atomic numbers:

• Elements numbered in blue are liquids at room temperature;
• those in green at gases at room temperature;
• those in black are solid at room temperature;
• those in red are synthetic and do not occur naturally (all are solid at room temperature).
• those in gray have not yet been discovered (they also have muted fill colors indicating the likely chemical series they would fall under).

And here is the periodic table (http://bmrl.med.uiuc.edu:8080/MRITable/MRItable.html) for magnetic resonance.

The number of electron shells an atom has determines what period it belongs to. 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 doesn't 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.

See also:

• Periodic table group
• Periodic table period
• Chemical series
• Periodic table block

• Isotope table (complete)
• Isotope table (divided)

• Discovery of the chemical elements
• Elements song
• IUPAC's systematic element names.

External links

• A Brief History of the Development of Periodic Table (http://www.wou.edu/las/physci/ch412/perhist.htm)
• Chemistry: Web Elements (http://www.webelements.com)
• Periodic Table Game (http://quizhub.com/quiz/periodictable.cfm)
• The Periodic Table of Comic Books (http://www.uky.edu/Projects/Chemcomics/) at the Department of Chemistry, University of Kentucky
• List of Periodic Table Elements that can be sorted by physical characteristics (http://www.science.co.il/PTelements.asp)
• Periodic table with mineral emphasis (http://webmineral.com/chemical.shtml)