Chemical symbol

Chemical symbols are the international standard way to denote chemical elements. In particular, they are used in chemical formulas to describe the composition and structure of molecules, and in reaction formulas. A chemical symbol consists of one or two letters — the initial letter of its scientific name in uppercase which, in most cases, is followed by a suitable lowercase letter from the name. (Sometimes, on a temporary basis, for new artificial elements initially 3-letter symbols may be used.)

Well-known examples of one-letter symbols are H for hydrogen and O for oxygen, Ca is the two-letter symbol for calcium. For most elements the symbol fits its English name because this name is also derived from the scientific name (which usually is of Greek or Latin origin). However, in a few cases there is no relation between the English word and its symbol. For instance, Fe (derived from Latin "ferrum") is the symbol for iron.

Subscripts and superscripts attached to the basic symbol are used to carry additional information.

Compounds

The number of atoms in a molecule is indicated by a subscript (on the right). For example, O₂ is oxygen, and O₃ ozone.

Ions

An Ion of an element is indicated by a superscript, where + and - stand for positive and negative charge, respectively. For example, H⁺ means a hydrogen ion, and Ca²⁺ a calcium ion (with two electrons missing).
The ion H⁺ of hydrogen is a proton which — in atomic reactions — is also indicated as p.

Isotopes

In atom physics, isotopes of an element are distinguished by adding the atomic mass as a superscript (usually, but not always, attached to the left of the symbol).
The isotopes of hydrogen have symbols of their own: ¹H is hydrogen H, ²H is deuterium D, and ³H is tritium T.

Examples

A simple example for a chemical reaction formula is

2 H₂ + O₂ → 2 H₂O

which states that hydrogen and oxygen can react and produce water.

H⁺ + OH^- = H₂O

²D + ²D = ³T + p

Chemical symbols may also be changed to show if one particular isotope of an atom that is specified, as well as to show other attributes such as the ionization and oxidation state of a chemical compound.

Attached subscripts or superscripts specifying a nucleotide or molecule have the following meanings and positions:

• The number of atoms of a nucleotide is shown in the right subscript position (e.g., N₂)
• The proton number (atomic number) may be indicated in the left subscript position (e.g., ₆₄Gd)