Chemical Symbol

A chemical symbol is a letter or pair of letters that represents a specific chemical element. For example, H represents hydrogen, O represents oxygen, Fe represents iron. Symbols are used to write chemical formulas and reaction equations instead of spelling out the full name.

The notation follows a consistent rule: the first letter is capitalized, and the second letter, if present, is lowercase. This distinguishes Co (cobalt) from CO (carbon monoxide, a compound of two elements). Symbols are identical across languages, although the names of the elements differ in each language.

History of symbols

The system of letter symbols was introduced by Swedish scientist Jöns Jacob Berzelius in the early 19th century. Before this, pictorial symbols derived from alchemy were used. Berzelius took the symbols from Latin names of the elements, which explains why iron is Fe (ferrum), silver is Ag (argentum), gold is Au (aurum), and copper is Cu (cuprum).

Use in formulas

Chemical formulas add subscript numbers to symbols showing how many atoms of each element are in a molecule: H₂O contains two hydrogen atoms and one oxygen atom. A superscript number indicates the number of molecules. Other symbols such as + and − show the electrical charge of ions; for example, Na⁺ and Cl⁻. The state of matter is sometimes shown with lowercase letters in parentheses: (s), (l), (g), (aq).

Atomic number and isotopes

To distinguish isotopes, the mass number is written as a superscript on the upper left, and the atomic number as a subscript on the lower left. This way ordinary carbon and carbon-14 (radioactive) are distinguished.

Temporary symbols

Newly discovered elements that have not yet been officially named are given three-letter temporary symbols based on their atomic number; for example, Uue for element 119. When the International Union of Pure and Applied Chemistry (IUPAC) approves an official name, it is replaced with a permanent symbol of one or two letters.