Formula unit

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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.

See also

Individual evidence

  1. a b Karl-Heinz Lautenschlager, Werner Schroeter, Joachim Teschner, Hildegard Bibrack, Handbook of Chemistry , 18th Edition, Harri German, Frankfurt (Main)., 2001
  2. Entry on chemical sign language. In: Römpp Online . Georg Thieme Verlag, accessed on June 20, 2014.
  3. Hans Rudolf Christen : Fundamentals of general and inorganic chemistry , Otto Salle, Frankfurt a. M., Sauerländer, Aarau, 9th edition, 1988, p. 257.