Chemical symbol

A chemical symbol is an abbreviation or shortened version of the name of a chemical element, generally assigned in relation to its Latin name. Natural elements all have symbols of one or two letters; some man-made elements have temporary symbols of three letters. Each element is denoted by the first letter of its name in English or Latin in upper case (capitals), so hydrogen, nitrogen and oxygen are symbolically represented by H, N and O, respectively. But as the first letter of the name of several elements is the same, in these cases the element is represented by two letters: the first letter of the symbol is in capitals followed by the second letter in lower case, thus calcium and silicon are indicated by Ca and Si.

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⁺)