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Chemical element

2007 Schools Wikipedia Selection. Related subjects: General Chemistry

   A chemical element, often called simply an element, is a substance that
   cannot be decomposed or transformed into other chemical substances by
   ordinary chemical processes. All matter consists of these elements and
   as of 2006, 117 unique elements have been discovered or artificially
   created. The smallest particle of such an element is an atom, which
   consists of electrons centered about a nucleus of protons and neutrons.

Chemistry terminology

   Earlier an element or pure element was defined as a substance which
   "can't be further broken down into another compound with different
   chemical properties"—which should be taken to mean it consists of atoms
   of one element. However, because of allotropy, the isotope effect, and
   the confusion with the more useful term referring to the general class
   of atoms (irrespective of what compound it may be in), this usage is in
   disfavor amongst contemporary chemists, and sees restricted, mostly
   historical, use. This definition was motivated by the observation that
   these elements could not be dissociated by chemical means into other
   compounds. For example, water could be converted into hydrogen and
   oxygen, but hydrogen and oxygen could not be further decomposed, thus
   "elemental". There are also many counterexamples (for example
   "elemental oxygen" (O[2]) can be decomposed by solely chemical means
   into oxygen ions and atoms which have drastically different chemical
   properties). This article will concern itself with the latter
   definition.

Description

   The lightest elements are hydrogen and helium. All the heavier elements
   are made, both naturally and artificially, through various methods of
   nucleosynthesis. As of 2006, there are 117 known elements: 94 occur
   naturally on Earth (six in trace quantities: technetium, atomic number
   43; promethium, atomic number 61; astatine, atomic number 85; francium,
   atomic number 87; neptunium, atomic number 93; and plutonium, atomic
   number 94) and 95 (including californium) have been detected in the
   universe at large. The 23 elements not found on earth are derived
   artificially; 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. All artificially derived elements
   are radioactive with short half-lives, so if any atoms of these
   elements were present at the formation of Earth they are extremely
   likely to have already decayed.

   Lists of the elements by name, by symbol, by atomic number, by density,
   by melting point, and by boiling point as well as Ionization energies
   of the elements are available. The most convenient presentation of the
   elements is in the periodic table, which groups elements with similar
   chemical properties together.

Atomic number

   The atomic number of an element, Z, is equal to the number of protons
   which defines the element. For example, all carbon atoms contain 6
   protons in their nucleus, so for carbon Z=6. These atoms may have
   different amounts of neutrons, and are known as isotopes of the
   element. The atomic mass of an element, A, is measured in unified
   atomic mass units (u) is the average mass of all the atoms of the
   element in an environment of interest (usually the earth's crust and
   atmosphere). Since electrons are of negligible mass, and neutrons are
   barely more than the mass of the proton, this usually corresponds to
   the sum of the protons and neutrons in the nucleus of the most abundant
   isotope, though this is not always the case (notably chlorine, which is
   about three-quarters ^35Cl and a quarter ^37Cl).

Atomic mass

   The atomic masses that are given on the periodic table are actually the
   relative atomic masses, which are calculated by the following method.
   As an example, assume there exists three isotopes of element X and
   their respective atomic masses are 10, 20 and 30 AMU for sake of
   demonstration. Now also assume that 50% of the isotopes of element X
   are the 10 AMU version and the two heavier isotopes each account for
   25% of the total number of atoms (particles) of this hypothetical
   element. As a result 10 * 0.5 = 5 AMU and 20 * 0.25= 5 AMU and 30 *
   0.25 = 7.5 AMU. The average atomic mass that results is 17.5 AMU. The
   reason is because the method to calculate the average mass takes into
   account the relative abundance of all of the isotopes of an element,
   which is multiplied against their individual masses.

Isotopes

   Some isotopes are radioactive and decay into other elements upon
   radiating an alpha or beta particle. Some elements have no
   nonradioactive isotopes, in particular all elements with atomic numbers
   greater than 82.

Nomenclature

   The naming of elements precedes the atomic theory of matter, although
   at the time it was not known which chemicals were elements and which
   compounds. When it was learned, existing names (e.g., gold, mercury,
   iron) were kept in most countries, and national differences emerged
   over the names of elements either for convenience, linguistic niceties,
   or nationalism. For example, the Germans use "Wasserstoff" for
   "hydrogen" and "Sauerstoff" for "oxygen," while English and some
   romance languages use "sodium" for "natrium" and "potassium" for
   "kalium," and the French prefer the term "azote" for "nitrogen." This
   is also used by the Greeks.

   But for international trade, the official names of the chemical
   elements both ancient and recent are decided by the International Union
   of Pure and Applied Chemistry, which has decided on a sort of
   international English language. That organization has recently
   prescribed that "aluminium" and "caesium" take the place of the US
   spellings "aluminium" and "cesium," while the US "sulfur" takes the
   place of the British "sulphur." But chemicals which are practicable to
   be sold in bulk within many countries, however, still have national
   names, and those which do not use the Latin alphabet cannot be expected
   to use the IUPAC name. According to IUPAC, the full name of an element
   is not capitalized, even if it is derived from a proper noun (unless it
   would be capitalized by some other rule, for instance if it begins a
   sentence).

   In the second half of the twentieth century physics laboratories became
   able to produce nuclei of chemical elements that have a half life too
   short for them to remain in any appreciable amounts. These are also
   named by IUPAC, which generally adopts the name chosen by the
   discoverer. This can lead to the controversial question of which
   research group actually discovered an element, a question which delayed
   the naming of elements with atomic number of 104 and higher for a
   considerable time. (See element naming controversy).

   Precursors of such controversies involved the nationalistic namings of
   elements in the late nineteenth century. For example, lutetium was
   named in reference to Paris, France. The Germans were reluctant to
   relinquish naming rights to the French, often calling it cassiopeium.
   The British discoverer of niobium originally named it columbium, in
   reference to the New World. It was used extensively as such by American
   publications prior to international standardization.

Chemical symbols

Specific chemical elements

   Before chemistry became a science, alchemists had designed arcane
   symbols for both metals and common compounds. These were however used
   as abbreviations in diagrams or procedures; there was no concept of one
   atoms combining to form molecules. With his advances in the atomic
   theory of matter, John Dalton devised his own simpler symbols, based on
   circles, which were to be used to depict molecules.

   The current system of chemical notation was invented by Berzelius. In
   this typographical system chemical symbols are not used as mere
   abbreviations - though each consists of letters of the Latin alphabet -
   they are symbols intended to be used by peoples of all languages and
   alphabets. The first of these symbols were intended to be fully
   universal; since Latin was the common language of science at that time,
   they were abbreviations based on the Latin names of metals - Fe comes
   from Ferrum, Ag from Argentum. The symbols were not followed by a
   period (full stop) as abbreviations were. Later chemical elements were
   also assigned unique chemical symbols, based on the name of the
   element, but not necessarily in English. For example, sodium has the
   chemical symbol 'Na' after the Latin natrium. The same applies to "W"
   (wolfram) for tungsten, "Hg" (hydrargyrum) for mercury, "K" (kalium)
   for potassium, and "Sb" (stibium) for antimony.

   Chemical symbols are understood internationally when element names
   might need to be translated. There are sometimes differences; for
   example, the Germans have used "J" instead of "I" for iodine, so the
   character would not be confused with a roman numeral.

   The first letter of a chemical symbol is always capitalized, as in the
   preceding examples, and the subsequent letters, if any, are always
   lower case (small letters).

General chemical symbols

   There are also symbols for series of chemical elements, for comparative
   formulas. These are one capital letter in length, and the letters are
   reserved so they are not permitted to be given for the names of
   specific elements. For example, an "X" is used to indicate a variable
   group amongst a class of compounds (though usually a halogen), while
   "R" is used for a radical, meaning a compound structure such as a
   hydrocarbon chain. The letter "Q" is reserved for "heat" in a chemical
   reaction. "Y" is also often used as a general chemical symbol, although
   it is also the symbol of Yttrium. "Z" is also frequently used as a
   general variable group. "L" is used to represent a general ligand in
   inorganic and organometallic chemistry. "M" is also often used in place
   of a general metal.

Isotope symbols

   Although not officially used, in nuclear physics the three main
   isotopes of the element hydrogen are often written as H for protium, D
   for deuterium and T for tritium. This is in order to make it easier to
   use them in chemical equations, as it replaces the need to write out
   the AMU for each isotope. It is written like this:

   D[2]O ( heavy water)

   Instead of writing it like this:

   ^2H[2]O

Most common elements in the Universe

   These are the ten most common elements in the Universe as measured in
   parts per million:
    Element  Parts per million
   Hydrogen  739,000
   Helium    240,000
   Oxygen    10,700
   Carbon    4,600
   Neon      1,340
   Iron      1,090
   Nitrogen  970
   Silicon   650
   Magnesium 580
   Sulfur    440

Recently discovered elements

   Element 118, Ununoctium, the heaviest element found to date, was
   successfully created/synthesized (synonymous in this context) on
   October 9, 2006, by the Flerov Laboratory of Nuclear Reactions in
   Dubna, Russia

   Element 117, Ununseptium, is yet to be created or discovered, although
   its place in the periodic table is preestablished, and likewise for
   possible elements beyond 118.
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