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Wednesday 27 May 2009

chemical element


Definition : 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.

The term is also used to refer to a pure chemical substance composed of atoms with the same number of protons.

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.

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.

The lightest elements are hydrogen and helium, both theoretically created by Big Bang nucleosynthesis during the first 20 minutes of the universe in a ratio of around 3:1 by mass (approximately 12:1 by number of atoms).

Almost all other elements found in nature, including some further hydrogen and helium created since then, were made by various natural or (at times) artificial methods of nucleosynthesis, including occasionally breakdown activities such as nuclear fission, alpha decay, cluster decay, and cosmic ray spallation.

As of 2006, there are 117 known elements (in this context, "known" means observed well enough, even from just a few decay products, to have been differentiated from any other element).

Natural elements

Of these 117 elements, 94 occur naturally on Earth. Six of these occur in extreme trace quantities: technetium, atomic number 43; promethium, number 61; astatine, number 85; francium, number 87; neptunium, number 93; and plutonium, number 94. These 94 elements, and also possibly element 98 californium, have been detected in the universe at large, in the spectra of stars and also supernovae, where short-lived radioactive elements are newly being made.

Artificial elements

The remaining 22 elements, not found on Earth or in astronomical spectra, have been derived artificially.

All of the elements that are derived solely through artificial means are radioactive with very short half-lives; if any atoms of these elements were present at the formation of Earth, they are extremely likely to have already decayed, and if present in novae, have been in quantities too small to have been noted.

Technetium was the first purportedly non-naturally occurring element to be synthesized, in 1937, although trace amounts of technetium have since been found in nature, and the element may have been discovered naturally in 1925.

This pattern of artificial production and later natural discovery has been repeated with several other radioactive naturally occurring trace elements.

Periodic table

Lists of the elements are available by name, by symbol, by atomic number, by density, by melting point, and by boiling point as well as Ionization energies of the elements.

The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.

Main elements in biology

- carbon C
- oxygen O
- nitrogen N
- hydrogen H
- phosphor P
- iron F
- copper Cu