Periodic table

The Periodic Table of Mendeleev is a chemical elements classification that allows to recognize dependency of their different characteristic from the number of protons in the atomic nucleus. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring ("periodic") trends in the properties of the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

The periodic table is now ubiquitous within the academic discipline of chemistry, providing an extremely useful framework to classify, systematize, and compare all of the many different forms of chemical behavior. The table has found wide application in chemistry, physics, biology, and engineering, especially chemical engineering. The current standard table contains 117 elements as of July 2009 (elements 1–116 and element 118).