Elements

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Some examples of elements include gold, iron, copper,carbon, silicon, mercury, sodium, calcium, hydrogen,nitrogen, chlorine, and neon. 
As of now, 118 elements have been identified. Of the 118 known elements, only the first 94 are believed to occur naturally on Earth. Of these naturally occurring elements, 80 are stable or essentially so, while the others are radioactive. Additional elements, of higher atomic numbers than those naturally occurring, have been produced technologically in recent decades as the products of nuclear reactions. The properties of the chemical elements are often summarized using the periodic table that organizes the elements by increasing atomic number into rows ("periods") in which the columns ("groups") share recurring ("periodic") physical and chemical properties. Its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869.