A formula unit is a type of chemical formula for compounds that do not consist of individual molecules .
In inorganic substances based on ionic bonds , a huge number of positive and negative ions form an ion lattice . The formulas (formula units) given for this compound generally indicate the abbreviated ratio of the atoms involved in the elements in the compound and, at this point, correspond to a ratio formula .
However, the formula unit of a compound contains structural information and is sometimes referred to as a "resolved formula".
The formula units of simple salts such as sodium chloride or magnesium chloride are given in the form NaCl or MgCl 2 . The cations are given in the first place and the anions in the second place , which are present in the ion lattices as well as in aqueous solutions (Na + , Mg 2+ , Cl - ). In the case of more complex salts, which consist of polyatomic ions, the central atom and then its ligands are named for the ions , as in calcium phosphate Ca 3 (PO 4 ) 2 and ammonium sulfate (NH 4 ) 2 SO 4 with the complex ions (NH 4 ) + , (PO 4 ) 3− and (SO 4 ) 2− .
In the case of salts of inorganic acids that carry protons, the hydrogen atoms are treated like cations: Sodium dihydrogen phosphate is given as NaH 2 PO 4 . This is done in analogy to the usual formulas of inorganic acids, in which the hydrogen atoms are given first (e.g. H 3 PO 4 for phosphoric acid ), which often does not do justice to their molecular structure [P (O) (OH) 3 ] .
The term formula unit is intended to avoid the use of molecules or particles when dealing with chemical issues when there are no molecules or free particles at all.
- ↑ a b Karl-Heinz Lautenschlager, Werner Schroeter, Joachim Teschner, Hildegard Bibrack, Handbook of Chemistry , 18th Edition, Harri German, Frankfurt (Main)., 2001
- ↑ Entry on chemical sign language. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.
- ↑ Hans Rudolf Christen : Fundamentals of general and inorganic chemistry , Otto Salle, Frankfurt a. M., Sauerländer, Aarau, 9th edition, 1988, p. 257.