Chemical element

A chemical element, often called simply element, is the class of atoms which contain the same number of protons.

An element or pure element can also refer to a compound which "cannot 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, due to 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" (O2) can be decomposed by solely chemical means into oxygen ions and atoms which have drastically different chemical properties).

The remainder of this article will concern itself with the first definition.

Characterization

The atomic number of an element or atom, Z, is equal to the number of protons. This is what defines the element. For example, all carbon atoms contain 6 protons in their nucleus, so for carbon Z=6. The mass number of an atom, A, measured in atomic mass units (amu), is the the sum of the number of protons and neutrons in the nucleus. One proton or one neutron is assigned a mass number of one since electrons are light, and neutrons are barely more than the mass of the proton. Now atoms of the same element can have different numbers of neutrons. Atoms with same atomic number, but different mass numbers are known as isotopes of the element. For an element the weighted average atomic mass, in amu, of all the naturally occuring isotopes is recorded in the periodic table. This usually corresponds to of the most abundant isotope, though this is not always the case (notably chlorine, which is about three-quaters 35-Cl and a quater 37-Cl).

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 Z >= 84.

The lightest elements are hydrogen and helium. Hydrogen is thought to be the first element to appear after the Big Bang. All the heavier elements, are made naturally and artificially through various methods of nucleosynthesis. There are as of 2005, 116 known elements, only 90 of which occur naturally. The remaining 26 are derived artificially; the first such element being technetium in 1937. All artificially derived elements are radioactive with short half-lives so that any such atoms that were present at the formation of Earth are extremely likely to have already decayed. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.

Elements are usually divided into three types; metals, non-metals, and metalloids. Metals tend to have properties such as metallic bonding, heat and electrical conductivity, and they are ductile (malleable). Whereas non-metals tend to exist as covalently bonded molecules and are insulators. There are comparitively few metalloids and they have properties which are intermediate between metals and non-metals. Metalloids tend to exist as covalently bonded lattice structures which may conduct heat, but not electricity. They straddle a "staircase" boundary between the metals and the non-metals.

Though we recognize three types of elements the major division is between metals and non metals. In addition to different physical properties the chemical properties of the two groups are distinctive. Metals tend to loose electrons and the non-metals tend to gain electrons. Both seek the stable electronic configuration of a noble gas.

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 some romance languages use "natrium" for "sodium" and "kalium" for "potassium," and the French prefer the obsolete but historic term "azote" for "nitrogen."

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 "aluminum" 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).

And in the second half of the twentieth century physics laboratories became able to produce nuclei of chemical elements that have too quick a decay rate to ever be sold in bulk. 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 (e.g., as "leutitium" refers to Paris, France, the Germans were reticent about relinquishing naming rights to the French, often calling it "cassiopium"). And notably, the British discoverer of "niobium" originally named it "columbium," after the New World, though this did not catch on in Europe. In the late twentieth century, the Americans had to accept the international name just when it was becoming an economically important material.

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. These were superseded by the current typographical system in which chemical symbols are not used as mere abbreviations though each consists 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 international, for they were based on the Latin abbreviations of the names of metals: Fe comes from Ferrum; Ag from Argentum. The symbols were not followed by a period (full stop) as abbreviations were. Besides a name, later chemical elements are also given a unique chemical symbol, based on the name of the element, not necessarily derived from the colloquial English name. (e.g., sodium has chemical symbol 'Na' after the Latin natrium).

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 minuscule (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 (not to be confused with radical_(chemistry), 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.

Nonelement symbols

Nonelements, especially in organic and organometallic chemistry, often acquire symbols which are inspired by the elemental symbols. A few examples:

Cy - cyclohexyl; Ph - phenyl; Bz - benzoyl; Bn - benzyl; Cp - Cyclopentadiene; Pr - propyl; Me - methyl; Et - ethyl; Tf - triflate; Ts - tosyl.

See also

• Chemistry
• Discovery of the chemical elements
• Abundance of the chemical elements
• Systematic element name
• Chemical elements named after people
• Chemical elements named after places
• Compound
• Periodic table

External links

• Elementymology & Elements Multidict word history and language dictionary

Chemical information

• WebElements
• ChemGlobe
• Los Alamos National Laboratory
• ChemicalElements

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