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.

Chemical symbols are listed in the periodic table and are used as shorthand in chemical formulas, chemical structures, and often in chemical equations, e.g.,

2 H₂ + O₂ → 2 H₂O

Because chemical symbols are often derived from the Latin or Greek name of the element, they may not bear much similarity to the common English name, e.g., Na for sodium (Latin natrium) and Au for gold (Latin aurum).

In China, each chemical element is assigned an ideograph as its symbol; most of them have been explicitly created for this purpose (see Chinese characters for chemical elements).

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 nucleon number (mass number) is shown in the left superscript position (e.g., ¹⁴N)
• 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)
• If necessary, a state of ionization or an excited state may be indicated in the right superscript position (e.g., state of ionization Na⁺)