A chemical formula describes the composition of chemical compounds and can contain information about their structure. A chemical formula always contains information on the chemical elements contained in a compound and the numerical ratio of the particles occurring in the compound. Chemical formulas are also used to represent chemical reactions in reaction equations.
The element symbols must be distinguished from the formulas : H for hydrogen , Mg for magnesium , N for nitrogen . If two or more atoms of the same element combine to form a molecule , this is in turn characterized by a formula: H 2 for hydrogen, N 2 for nitrogen, O 3 for ozone .
There are different ways of writing chemical formulas. The best known are among others:
- Ratio formula : It is made up of element symbols and small , subscript numbers (indices) and simply reflects the ratio in which the individual chemical elements are contained in a chemical compound . Integer ratios are used here.
Molecular formula : The molecular formula indicates the number of atoms of the chemical elements that are contained in a compound . In the case of molecules , this also indicates the actual frequency of the atoms of different elements in the molecule. The empirical formulas of salts generally correspond to their ratio formulas , but are usually given in the form of formula units which satisfy the stoichiometry of the compound.
- Example: The molecular formula for benzene is C 6 H 6 , the molecular formula (formula unit) for aluminum sulfate is Al 2 (SO 4 ) 3 .
- Structural formula : In addition to the number of different atoms in a molecule , the structural formula also shows their arrangement to one another, i.e. it also provides information about the type of atoms and the type of their chemical bonds , possibly also about bond angles and bond lengths . The letters stand for individual atoms. A line corresponds to a pair of electrons that is present in an electron cloud / orbital and either forms the bond between atoms or is a so-called free electron pair. Correspondingly, two parallel lines mean a double bond .
Crystal chemical structural formula : It is a special formula notation that is used in mineralogy for more complex chemical compounds . The crystal chemical structural formula is based on the empirical formula , but sorts the components according to cations and anions as well as the structural position of the anions.
- Example: The molecular formula for fluoroapatite is Ca 5 (PO 4 ) 3 F, the crystal chemical structural formula on the other hand Ca 5 [F | (PO 4 ) 3 ], since the
The use of the names of the individual formula types is not always standardized and many names are used synonymously. Structural formulas are mainly used to describe molecules . However, a structural formula is also used to describe ions in crystals , since the ions in the ion crystals are also "structured". A formula used for salt- like substances should be referred to as a formula unit , not a structural formula or empirical formula . The actual structure of a salt-like compound is only clarified by its crystal structure .
Comparison of different formulas with different degrees of abstraction
|Structural formulas||Other modes of representation|
|Electron formula||Valence stroke formula||Wedge formula||Skeletal formula||Constitutional formula||Molecular formula||Ratio formula|
|methane||does not exist||CH 4||CH 4||CH 4|
|propane||CH 3 -CH 2 -CH 3||C 3 H 8||C 3 H 8|
|acetic acid||CH 3 -COOH||C 2 H 4 O 2||CH 2 O|
|water||does not exist||H 2 O||H 2 O||H 2 O|
Further display options
- Constitutional formula
- Fischer projection formula : for chiral molecules such as amino acids or sugars
- Haworth projection , for 5- to 6-ring molecules such as sugar
- Newman projection formula : to represent the conformation
- Structure codes: International Chemical Identifier , Simplified Molecular Input Line Entry Specification , Wiswesser Line Notation