Why these ionic compounds don't exist
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Transitions between ionic and covalent bonds
The extreme cases of largely ionic and purely covalent bonds are actually only realized in relatively few examples. In the case of homonuclear diatomic molecules such as,, and, one can speak of a purely covalent bond. The electron distribution between the atoms is completely symmetrical. The most pronounced examples of ionic bonding are provided by the compounds of metals with low ionization energy and non-metals with high electron affinity; these are the alkali and alkaline earth halides and oxides. Most connections fall somewhere between the two extremes. A clear indication of this is the polarity of atomic bonds.
If one imagines the approach of two spherical ions with opposite charges, the cation attracts the electron shell of the anion and deforms (polarizes) it to a greater or lesser extent. If the polarization of the anion increases, the bond becomes increasingly covalent. The electron density between the binding partners increases and the ionic bond character decreases.
The ability of a cation to polarize the electron shell of a neighboring anion depends on the charge concentration on the cation. Small and highly charged cations polarize the most. On the other hand, large multiply charged anions are easiest to polarize because the outermost electrons are far away from the nucleus. As a result, small and highly charged cations in compounds as such cannot exist at all, because the polarization of the anion leads to an electron density between the binding partners and thus to at least partially covalent bonds.
So are the metal-non-metal compounds and not salt-like substances, but liquids. Quadruple charged cations do not exist because their strongly polarizing effect leads to polarized covalent bonds, i.e. the compounds mentioned are molecular. The same applies to the solid but easily sublimable compounds, and. Metal-non-metal compounds do not always have an ionic character!
The problem of bonds of mixed character can also be viewed from the point of view of the covalent bond, namely as the polarization of the bonding electrons towards the more electronegative partner.
The extent of the polarization of the binding electrons is determined by the electronegativity difference of the binding partners. In the case of lithium fluoride, there is no longer a pair of binding electrons; it has, so to speak, been transferred to the more electronegative fluorine.
The strong interactions in ionic substances (electrostatic attraction of the ions) contrasts with a relatively weak intermolecular interaction in substances made up of molecules. The different strengths of the interactions are expressed in the very different macroscopic properties of the substances such as hardness or volatility. However, substances with a covalent bond appear not only as molecules, but also as covalent solids with high melting points (e.g. diamond or quartz).
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