Editing Chemical element (section)

== Nomenclature and symbols ==
{{anchor|Nomenclature}}

The various chemical elements are formally identified by their unique atomic numbers, their accepted names, and their [[chemical symbol]]s.

=== Atomic numbers ===
The known elements have atomic numbers from 1 to 118, conventionally presented as [[Arabic numerals]]. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest (as in a periodic table), sets of elements are sometimes specified by such notation as "through", "beyond", or "from ... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities), as in "lighter than carbon" or "heavier than lead", though the atomic masses of the elements (their atomic weights or atomic masses) do not always increase [[monotonic function|monotonically]] with their atomic numbers.

=== Element names ===
{{main|Naming of chemical elements}}

The naming of various substances now known as elements precedes the [[atomic theory]] of matter, as names were given locally by various cultures to various minerals, metals, compounds, alloys, mixtures, and other materials, though at the time it was not known which chemicals were elements and which compounds. As they were identified as elements, the existing names for anciently known elements (e.g., gold, mercury, iron) were kept in most countries. National differences emerged over the element names either for convenience, linguistic niceties, or nationalism. For example, German speakers use "Wasserstoff" (water stuff) for "hydrogen", "Sauerstoff" (acid stuff) for "oxygen", and "Stickstoff" (smothering stuff) for "nitrogen"; English and some other languages use "sodium" for "natrium", and "potassium" for "kalium"; and the French, Italians, Greeks, Portuguese and Poles prefer "azote/azot/azoto" (from roots meaning "no life") for "nitrogen".

For purposes of international communication and trade, the [[Table of chemical elements|official names of the chemical elements]] both ancient and more recently recognised are decided by the [[International Union of Pure and Applied Chemistry]] (IUPAC), which has decided on a sort of international English language, drawing on traditional English names even when an element's chemical symbol is based on a Latin or other traditional word, for example adopting "gold" rather than "aurum" as the name for the 79th element (Au). IUPAC prefers the British spellings "[[aluminium]]" and "caesium" over the U.S. spellings "aluminum" and "cesium", and the U.S. "sulfur" over British "sulphur". However, elements that are practical to sell in bulk in many countries often still have locally used national names, and countries whose national language does not use the [[Latin alphabet]] are likely to use the IUPAC element names.

According to IUPAC, element names are not proper nouns; therefore, the full name of an element is not capitalised in English, even if derived from a [[proper noun]], as in [[californium]] and [[einsteinium]]. Isotope names are also uncapitalised if written out, ''e.g.,'' [[carbon-12]] or [[uranium-235]]. Chemical element ''symbols'' (such as Cf for californium and Es for einsteinium), are always capitalised (see below).

In the second half of the 20th century, physics laboratories became able to produce elements with half-lives too short for an appreciable amount of them to exist at any time. These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This practice can lead to the controversial question of which research group actually discovered an element, a question that delayed the naming of elements with atomic number of 104 and higher for a considerable amount of time. (See [[element naming controversy]]).

Precursors of such controversies involved the nationalistic namings of elements in the late 19th century. For example, ''[[lutetium]]'' was named after Paris, France. The Germans were reluctant to relinquish naming rights to the French, often calling it ''cassiopeium''. Similarly, the British discoverer of ''[[niobium]]'' originally named it ''columbium'', in reference to the [[New World]]. It was used extensively as such by American publications before the international standardisation (in 1950).

=== Chemical symbols ===
{{For|lists of current chemical symbols, symbols not currently used, and other symbols that may look like chemical symbols|Chemical symbol}}

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

The current system of chemical notation was invented by [[Jöns Jacob Berzelius]] in 1814. In this system, chemical symbols are not mere abbreviations—though each consists of letters of the [[Latin alphabet]]. They are intended as universal symbols for people of all languages and alphabets.

Since Latin was the common language of science at Berzelius' time, his symbols were abbreviations based on the [[Latin]] names of elements (they may be Classical Latin names of elements known since antiquity or [[Neo-Latin]] coinages for later elements). The symbols are not followed by a period (full stop) as with abbreviations. In most cases, Latin names of elements as used by Berzelius have the same roots as the modern English name. For example, [[hydrogen]] has the symbol "H" from Neo-Latin {{lang|la|hydrogenium}}, which has the same Greek roots as English ''hydrogen''. However, in eleven cases Latin (as used by Berzelius) and English names of elements have different roots. Eight of them are the seven [[metals of antiquity]] and a metalloid also known since antiquity: "Fe" (Latin {{lang|la|ferrum}}) for [[iron]], "Hg" (Latin {{lang|la|hydrargyrum}}) for [[Mercury (element)|mercury]], "Sn" (Latin {{lang|la|stannum}}) for [[tin]], "Au" (Latin {{lang|la|aurum}}) for gold, "Ag" (Latin {{lang|la|argentum}}) for [[silver]], "Pb" (Latin {{lang|la|plumbum}}) for [[lead]], "Cu" (Latin {{lang|la|cuprum}}) for [[copper]], and "Sb" (Latin {{lang|la|stibium}}) for [[antimony]]. The three other mismatches between Neo-Latin (as used by Berzelius) and English names are "Na" (Neo-Latin {{lang|la|natrium}}) for [[sodium]], "K" (Neo-Latin {{lang|la|kalium}}) for [[potassium]], and "W" (Neo-Latin {{lang|la|wolframium}}) for [[tungsten]]. These mismatches came from different suggestings of naming the elements in the [[Modern era]]. Initially Berzelius had suggested "So" and "Po" for sodium and potassium, but he changed the symbols to "Na" and "K" later in the same year.

Elements discovered after 1814 were also assigned unique chemical symbols, based on the name of the element. The use of Latin as the universal language of science was fading, but chemical names of newly discovered elements came to be borrowed from language to language with little or no modification. Symbols of elements discovered after 1814 match their names in English, French (ignoring the [[acute accent]] on ⟨é⟩), and German (though German often allows alternate spellings with ⟨k⟩ or ⟨z⟩ instead of ⟨c⟩: e.g., the name of [[calcium]] may be spelled {{lang|de|Calcium}} or {{lang|de|Kalzium}} in German, but its symbol is always "Ca"). Other languages sometimes modify element name spellings: Spanish {{lang|es|iterbio}} (ytterbium), Italian {{lang|it|afnio}} (hafnium), Swedish {{lang|sv|moskovium}} (moscovium); but those modifications do not affect chemical symbols: Yb, Hf, Mc.

Chemical symbols are understood internationally when element names might require translation. There have been some differences in the past. For example, Germans in the past have used "J" (for the name {{lang|de|Jod}}) for iodine, but now use "I" and {{lang|de|Iod}}.

The first letter of a chemical symbol is always capitalised, and the subsequent letters, if any, are always lowercase; see the preceding examples.

==== General chemical symbols ====
There are also symbols in chemical equations for groups of elements, for example in comparative formulas. These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, "'''X'''" indicates a variable group (usually a halogen) in a class of compounds, while "'''R'''" is a [[Radical (chemistry)|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, though it is also the symbol of [[yttrium]]. "'''Z'''" is also often used as a general variable group. "'''E'''" is used in organic chemistry to denote an [[Polar effect|electron-withdrawing group]] or an [[electrophile]]; similarly "'''Nu'''" denotes a [[nucleophile]]. "'''L'''" is used to represent a general [[ligand]] in [[inorganic chemistry|inorganic]] and [[organometallic chemistry]]. "'''M'''" is also often used in place of a general metal.

At least two other, two-letter generic chemical symbols are also in informal use, "'''Ln'''" for any [[lanthanide]] and "'''An'''" for any [[actinide]]. "'''Rg'''" was formerly used for any [[rare gas]] element, but the group of rare gases has now been renamed [[noble gas]]es and "'''Rg'''" now refers to [[roentgenium]].

==== Isotope symbols ====
Isotopes of an element are distinguished by mass number (total protons and neutrons), with this number combined with the element's symbol. IUPAC prefers that isotope symbols be written in superscript notation when practical, for example {{sup|12}}C and {{sup|235}}U. However, other notations, such as carbon-12 and uranium-235, or C-12 and U-235, are also used.

As a special case, the three naturally occurring isotopes of hydrogen are often specified as '''H''' for {{sup|1}}H ([[protium]]), '''D''' for {{sup|2}}H ([[deuterium]]), and '''T''' for {{sup|3}}H ([[tritium]]). This convention is easier to use in chemical equations, replacing the need to write out the mass number each time. Thus, the formula for [[heavy water]] may be written D{{sub|2}}O instead of {{sup|2}}H{{sub|2}}O.