Chemical element

A chemical element is a type of atom that is distinguished by its atomic number; that is, by the number of protons in its nucleus. The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.[1] Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen. In total, 117 elements have been observed as of 2008, of which 94 occur naturally on Earth. 80 elements have stable isotopes (namely all elements with atomic numbers 1 to 82, except elements 43 and 61 (technetium and promethium). Elements with atomic numbers 83 or higher (bismuth and above) are inherently unstable, and undergo radioactive decay. The elements from atomic number 83 to 94 have no stable nuclei, but are nevertheless found in nature, either surviving as remnants of the primordial stellar nucleosynthesis which produced the elements in the solar system, or else produced as short-lived daughter-isotopes through the natural decay of uranium and thorium.[2]

All chemical matter consists of these elements. New elements of higher atomic number are discovered from time to time, as products of artificial nuclear reactions.

History
Mendeleev's 1869 periodic table

Several old philosophies used a set of archetypal classical elements to explain patterns in nature. The term 'element' was originally used to refer to a state of matter (solid/earth, liquid/water, gas/air, and plasma/fire) or a phase of matter (as in the Chinese Wu Xing), rather than the chemical elements of modern science. The Greek, Indian (Tattva and Mahābhūta) and Japanese (go dai) traditions essentially had the same five elements: Air, Earth, Fire, Water and Aether.

The term 'elements' (stoicheia) was first used by the Greek philosopher Plato in about 360 BCE, in his dialogue Timaeus, which includes a discussion of the composition of inorganic and organic bodies and is a rudimentary treatise on chemistry. Plato assumed that the minute particle of each element corresponded to one of the regular polyhedra: tetrahedron (fire), octahedron (air), icosahedron (water), and cube (earth).[citation needed][3]

Adding to the four elements of the Greek philosopher Empedocles, in about 350 BC, Aristotle also used the term "element"and conceived of a fifth element called "quintessence", which formed the heavens. Aristotle defined an element as:

Element – one of those bodies into which other bodies can be decomposed and which itself is not capable of being divided into other.