According to a classic definition due to L. Pauling there is a chemical bond between two atoms when
A fundamental characteristic of the ionic bond is the transfer of electrons from one atom to another, a fundamental characteristic of the covalent bond is the pooling of electrons between different atoms.
However, the aforementioned subdivision should not be rigorously interpreted, in fact, although each type of bond has well-defined characteristics, in many cases intermediate bonds can be formed, with a gradual transition from one extreme to the other. This in particular occurs in the case of ionic and covalent bonds.
Before moving on to a discussion, albeit brief, of the various types of bonds and their fundamental characteristics, let us preface some rules and some concepts that are of fundamental importance for understanding the mechanism of formation of the bonds themselves.
Although fundamentally the understanding of the chemical bond requires concepts of quantum physics, it must be emphasized that the chemical bond is an interaction between atoms that occurs through forces of an electrostatic nature.
The formation of the covalent bond in the hydrogen molecule occurs because the electron that rotates around an isolated hydrogen atom is affected by the attraction of a single nucleus (1 proton), whereas when the bond is formed it rotates in the atomic orbital of the molecule and affected by the attraction of two nuclei.
The same is true for the second electron of the second hydrogen atom. This leads to a situation of greater electrical stability, which consequently creates an approach of the two nuclei as long as the repulsive interactions between them are not significant. This can be better appreciated by plotting the energy of the H-H bond as a function of the distance between the two atoms.
Chemical bond, II
The above drawing shows three typical situations in the interaction between two hydrogen atoms.
Starting from the right, at a great distance with respect to the atomic dimensions, there is only a weak interaction, at a distance close to the binding one, the attractive interactions between the two electrons and the two nuclei stabilize the molecule, while finally at close range the repulsions between the nuclei destabilize the system.
Energy curve ?? interatomic distance for the H2 molecule
Analogous to an ionic-type bond, as in the compound Li-F, hydrogen fluoride, an electron is transferred from the lithium atom to the fluorine atom, after which the charges of opposite sign that are established on the two atoms create a intense electrostatic attraction that stabilizes the bond. This likewise creates an approach between the two atoms up to a certain distance, when the repulsive forces between the two nuclei and between the other external electrons of the atoms prevail.
Representation of valence electrons
The electrons of the outermost layer are fundamentally involved in the formation of chemical bonds, which for this reason are also called bond electrons or valence electrons. For an immediate visual representation of these electrons a method introduced by G. N. Lewis is often used: the symbol of the atom is surrounded by as many dots as there are valence electrons; two dots side by side indicate the two electrons of the same orbital, an isolated dot an unpaired, solitary electron in an orbital.
The nitrogen atom (1s2 2s2 2p3) is thus represented with five dots, two of which are coupled while the chlorine atom (1s2 2s2 2p6 3s2 3p5) is represented with the chemical symbol (Cl) surrounded by three more pairs of dots one isolated. According to this representation the letters N and Cl indicate the nucleus of the atom plus all the non-valence electrons, ie the so-called atom core.
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