Chemical element
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A chemical element, often called simply element, is a substance that cannot be divided or changed into different substances by ordinary chemical methods. The smallest particle of such an element is an atom, which consists of electrons centered on a nucleus of protons and neutrons.
The atomic number of an element, Z, is equal to the number of protons in the atom of the element. For example, carbon, the element with atomic number 6, contains 6 protons in its nucleus. All atoms of an element have the same atomic number and contain the same number of protons. However, atoms of the same element may differ in the number 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) and is roughly equal to the sum of the protons and neutrons in the nucleus of an atom of the element. A number of elements are radioactive and in undergoing radioactive decay transmute into a different element.
The lightest elements are hydrogen and helium, which were the first elements to appear in the Big Bang. All the heavier elements are made naturally and artificially through various methods of nucleosynthesis.
There are as of 2004, 116 known elements, only 91 of which occur naturally. The remaining 25 are derived artificially; the first such element being Technetium in 1937. All artificially derived elements are radioactive with short half-lives so that any that were present at the formation of Earth have long since decayed.
Lists of the elements by name, by symbol, and by atomic number are available. The most convenient presentation of the elements is in the periodic table, which groups elements with similar chemical properties together.
Atoms of the same element whose nuclei contain a different number of neutrons are said to be different isotopes of the element. A pure element can exist as monatomic units or as diatomic or polyatomic units comprising the same kind of atoms. These are called allotropes, irrespective of the state.
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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 any of the halogen elements, 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.
Reactivity
Elements can combine (react) to form pure compounds (such as water, salts, oxides and organic compounds). In many cases these compounds have essentially one fixed stoichiometry (composition) and their own structure and properties.
Some elements, particularly metallic elements, combine to form new structures with a more variable composition (such as metal alloys). In those cases it is better to speak of phases rather than compounds.
In general, a particular chemical can consist of a mixture of all of the above.