Periodic table

The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons), electron configurations, and recurring chemical properties. This ordering shows periodic trends, such as elements with similar behavior in the same column. It also shows four rectangular blocks with some approximately similar chemical properties. In general, within one row (period) the elements are metals on the lefthand side, and non-metals on the righthand side.

The rows of the table are called periods; the columns are called groups. Six groups (columns) have names as well as numbers: for example, group 17 elements are the halogens; and group 18, thenoble gases. The periodic table can be used to derive relationships between the properties of the elements, and predict the properties of new elements yet to be discovered or synthesized. The periodic table provides a useful framework for analyzing chemical behavior, and is widely used in chemistry and other sciences.

Dmitri Mendeleev published in 1869 the first widely recognized periodic table. He developed his table to illustrate periodic trends in the properties of the then-known elements. Mendeleev also predicted some properties of then-unknown elements that would be expected to fill gaps in this table. Most of his predictions were proved correct when the elements in question were subsequently discovered. Mendeleev's periodic table has since been expanded and refined with the discovery or synthesis of further new elements and the development of new theoretical models to explain chemical behavior.

All elements from atomic numbers 1 (hydrogen) to 118 (ununoctium) have been discovered or synthesized, with the most recent additions (elements 113, 115, 117, and 118) being confirmed by theIUPAC on December 30, 2015.^[1] The first 94 elements exist naturally, although some are found only in trace amounts and were synthesized in laboratories before being found in nature.^{[n 1]} Elements with atomic numbers from 95 to 118 have only been synthesized in laboratories. It has been shown that elements 95 to 100 once occurred in nature but currently do not.^[2] Synthesis of elements having higher atomic numbers is being pursued. Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories.

Contents

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• 1Overview
• 2Grouping methods
  • 2.1Groups
  • 2.2Periods
  • 2.3Blocks
  • 2.4Metals, metalloids and nonmetals
• 3Periodic trends
  • 3.1Electron configuration
  • 3.2Atomic radii
  • 3.3Ionization energy
  • 3.4Electronegativity
  • 3.5Electron affinity
  • 3.6Metallic character
• 4History
  • 4.1First systemization attempts
  • 4.2Mendeleev's table
  • 4.3Second version and further development
• 5Different periodic tables
  • 5.1Group 3 constitution variants
  • 5.2Periodic tables by different structure
• 6Open questions and controversies
  • 6.1Elements with unknown chemical properties
  • 6.2Further periodic table extensions
  • 6.3Element with the highest possible atomic number
  • 6.4Placement of hydrogen and helium
  • 6.5Groups included in the transition metals
  • 6.6Period 6 and 7 elements in group 3
  • 6.7Optimal form
• 7See also
• 8Notes
• 9References
• 10Bibliography
• 11External links