The periodic table is the systematic and tabular arrangement of all known chemical elements in which elements having similar properties kept together. In the table, the elements are placed in the order of their atomic numbers starting with the lowest number of one, hydrogen. The atomic number of an element is the same as the number of protons in that particular nucleus of an atom. In the periodic table the elements are arranged into periods and groups. A row of elements across the table is called a period. Each period has a number; from 1 to 8. Period 1 has only 2 elements in it: hydrogen and helium. Period 2 and Period 3 both have 8 elements. Other periods are longer. Elements in a period have consecutive atomic numbers.

A column of elements down the table is called a group. There are 18 groups in the standard periodic table.  Each group has a number: from 1 to 18. Elements in a group have electrons arranged in similar ways, according to the number of valency electrons, which gives them similar chemical properties (they behave in similar ways).  For example, group 18 is known as the noble gases because they are all gases and they do not combine with other atoms.

There are two systems of group numbers; one using Arabic numerals (1,2,3) and the other using Roman numerals (I, II, III). The Roman numeral names were used in most of the 20th century.  In 1990 the International Union of Pure and Applied Chemistry (IUPAC) decided to use the new system with Arabic numerals, to replace the two old group systems that used Roman numerals.

The periodic table has been used by chemists to observe patterns and relationships between elements. There are 3 main groups in the Periodic Table; metals, metalloids, and nonmetals. For example, elements to the bottom and far left of the table are the most metallic, and elements on the top right are the least metallic. (e.g. cesium is much more metallic than helium). There are also many other patterns and relationships.

The periodic table was invented by the Russian chemist Dmitry Ivanovich Mendeleyev (18341907). In his honor, element 101 was named after him, mendelevium.

Standard periodic table 

Chemical Series of the Periodic Table

 Alkali metals
 Alkaline earths
 Lanthanides
 Actinides
 Superactinides
 Transition metals
 Poor metals
 Metalloids/Semi metals/Half metals
 Nonmetals
 Halogens
 Noble gases

State at standard temperature and pressure. The color of the number (atomic number) above the element symbol shows the state of the element at normal conditions.
 those in blue are gases
 those in green are liquids
 those in black are solid

Radioactivity

 Those with solid borders have stable isotopes (Primordial elements)
 Those with dashed borders have only radioactive naturally occurring isotopes
 Those with dotted borders do not occur naturally (Synthetic Elements)
 Those without borders are too radioactive to have been discovered yet.

Other methods for displaying the chemical elements 
The version of the periodic table shown above is the one most used.  Other widespread versions are shown below:

Other versions 
 The standard table provides the basics. It is shown above
 A vertical table for improved readability 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
 Electron configurations
 Metals and Non Metals
 List of elements: includes name, symbol, atomic number, atomic mass, group, and period; sortable by any of those
 List of elements by symbol
 List of elements by boiling point
 List of elements by melting point
 List of elements by density

Related pages 
 Periodic table group
 Periodic table period
Extended periodic table

Other websites 

 Periodic Table of the Elements – displays basic properties
 WebElements Periodic table