Editing Chemical element (section)

=== 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.