How to calculate ionic compounds

Ionic bond

Estimation of the percentage of ion binding as a function of the electronegativity difference

The ionic bond (also Ionic bond, heteropolar bond or electrovalent bond) is a chemical bond based on the electrostatic attraction of positively and negatively charged ions.

description

The ionic bond was formulated by Walter Kossel around 1916. From an electronegativity difference of ΔEN = 1.7, one speaks of a 50% partially ionic character.[1] If the difference is greater than 1.7, then there are ionic bonds, including polar, predominantly covalent bonds. However, these are relatively arbitrary limits, since the case of the pure ionic bond represents an idealization. As a rough guide: There is an ionic bond between elements that are on the left in the periodic table (PSE) (i.e. metals) and elements that are on the right in the PSE (non-metals). If you look at the ionic binding content of sodium chloride, for example, which is often viewed as a classic case of ionic binding, you will find a value of around 75 percent. Another example would be cesium fluoride at about 92 percent. Ionic bonds therefore also have a share of covalent bonds in all cases. The reverse does not apply, because there is a 100 percent covalent bond within so-called element molecules.

Electron configuration

By accepting or releasing electrons, the atoms strive to achieve the noble gas configuration and the lowest energy state for their outermost occupied shell. This is achieved either through the release of electrons by the elements with lower electronegativity (left in the PSE), which creates single or multiple positively charged cations, or in the other case through electron uptake by the elements with higher electronegativity and thus high electron affinity (elements on the right in the PSE ), resulting in single or multiple negatively charged anions.

Formation of the ion lattice

The cations and anions attract each other electrostatically; the energy released when the two types of ions combine is called the lattice energy and is the actual driving force behind salt formation. The grid energy is made up of a total of 4 components:

  • the zero point energy of the ions,
  • the repulsion energies between the nuclei on the one hand and between the electron shells on the other,
  • the binding energy, which results from London forces between electron shells that are more or less easily polarizable or from multipole interactions (in the case of ions with an asymmetrical charge distribution such as NO2) results and
  • and finally the Coulomb force between the oppositely charged ions.

The lattice energy can be determined empirically using the Born-Haber cycle.

Lattice properties

Since the electrostatic field extends evenly in all spatial directions, very regular ion grids are created. Due to the different ionic radii, however, there are different ionic structures: common salt (NaCl), cesium chloride (CsCl), zinc blende (ZnS) and fluorite (CaF) structure2) as well as others named after the characteristic representatives. The relative stabilities of the different types of lattices due to different coordination geometries and coordination numbers of the ions are reflected by the Madelung constants; these are characteristic of the respective structure.

Characteristic properties of compounds with an ionic bond

  • High melting and boiling point, as the undirected bonding forces in crystals create a relatively stable bond across the entire crystal.
  • Conductive in the melt or in solution. The ions take over the charge transport. They are discharged at the electrodes, which breaks down the salts (often into their elements). This is why ion conductors are called second order conductors.
  • Hard and brittle: If you try to plastically deform a crystal, it usually bursts because the ions with the same charge are pushed towards each other in the crystal and the bond is thereby broken.
  • Crystal formation as a solid
  • Ion crystals are often colorless because the valence electrons are usually strongly bound and can only be excited by photons of higher energy than that of visible light.
  • Salts dissociate into their corresponding ions in aqueous solution; Ionic compounds dissolve in water - but to a very different extent. For example, sodium chloride is very soluble in water, while silver chloride is almost insoluble.

Individual evidence

  1. ^ Charles E. Mortimer, Ulrich Müller: Chemistry. The basic knowledge of chemistry. With exercises. 6th edition. Thieme Georg Verlag, 1996, ISBN 3-13-484306-4.

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