chemical formula

Different formulas of the benzene molecule.

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 .

Known formulas

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.
Example: The ratio formula for benzene is C ₁ H ₁ , the ratio formula for aluminum sulfate is Al ₂ O ₁₂ S ₃ .

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 .
There are different variations of the structural formula, for example, the Lewis structure , the valence bond , the Natta projection and the skeletal formula .

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 fluorine anion is not linked to the phosphate complex, but only to the calcium cation . Fluorine has been separated from the rest of the anion group and moved closer to the cation.

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