Chemical element

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.[1] Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen. All chemical matter consists of these elements.

Save for the hydrogen and helium in the universe, which are thought to have been mostly produced in the Big Bang, most chemical elements are thought to have been produced by later processes.

These processes are divided into:

* cosmic ray spallation (important for lithium, beryllium and boron, though some of these may have formed in the Big Bang), and
* stellar nucleosynthesis which produces all elements heavier than boron (with carbon being the first of this series). The very heaviest elements (those beyond element 94, plutonium) decay with half lives too short to allow them to be observed naturally on Earth.

In total, 118 elements have been characterized as of March 2010, and new elements of higher atomic number are "discovered" from time to time, as new synthetic products of artificial nuclear reactions. Of the known elements, the first 94 occur naturally on Earth. Of these, oxygen is the most abundant element in the Earth's crust. About 80 elements have stable isotopes: namely all elements with atomic numbers 1 to 82, except elements 43 and 61 (technetium and promethium). About half of the 80 stable elements are expected to be radioactive with such long half lives that their decay happens only in theory, and is too slow to have yet been detected by experiment (see the list of nuclides). These elements (such as bismuth, only recently measured as unstable) have half lives at least 100 million to 1000 million times longer than the estimated age of the universe.

Elements with atomic numbers 83 or higher (bismuth and above) are unstable to the point that their instability has been detected, and they undergo radioactive decay. The elements from atomic number 83 to 94 are composed entirely of radioactive isotopes. However, along with unstable elements 43 and 61, they are nevertheless found on Earth, though sometimes in very small amounts. Some of these elements, notably uranium and thorium, have one or more isotopes with half lives long enough to survive as remnants of the primordial explosive stellar nucleosynthesis that produced the heavy elements before formation of our solar system (see primordial nuclide). Other radioactive elements continue to be produced in natural processes, such as production by cosmic rays, or as shorter-lived daughter nuclides or transmutation products from natural decay of longer-lived radioactives

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