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The adjective covalent It is used in the field of chemistry to qualify the link that is generated between atoms that have shared electron pairs . It also qualifies as covalent to what has at least one covalent bond.

It is important to remember that particles that have an electric charge and are formed by a molecule or an atom that is not neutral are called ions . The ions, according to the octet rule that enunciated the American Gilbert Newton Lewis in 1916 , have the tendency to employ eight electrons to complete the latest energy levels and thus achieve stability in its configuration.

The atoms , to respect the octet rule, they can appeal to different kinds of chemical links to join. Among them appears the covalent bond , which is the electron sharing at the last level . This type of bond requires that the difference in electronegativity that is recorded between atoms be less than 1,7 .

Covalent bonds develop between atoms of different elements not metals and between atoms that belong to the same nonmetal element. Covalently bonded atoms share their electron pairs in the molecular orbital .

These atoms can share between one and three pairs of electrons in a covalent bond: therefore, the bonds can be simple , double or triple as appropriate If the bond occurs between equal atoms that have an electronegativity difference less than 0,4 , you get a apolar covalent bond . On the other hand, if the bond is developed by atoms of different elements that have an electronegativity difference greater than 0,4 , It's about a polar covalent bond .

According to the chemical G. William Daub and S. Seese, in any covalent substance (such as a molecule of hydrogen ) the following four aspects are appreciated:

* if they are observed individually, that is to say outside of a combination, the atoms possess very different properties to those that the molecules exhibit. For this reason, when writing the chemical formula of hydrogen, for example, we must put a two as subscript of the Hsince it is a diatomic molecule (the one that is formed by two atoms, whether or not they are the same chemical element );

* the two electrons are attracted to the two positive nuclei, something that happens with the objective of producing a more stable molecule than one in which the atoms are separated. This causes a covalent bond to be generated. Given that the attraction to which the nuclei subject the electrons manages to cancel out the repulsion between them, there is a high probability of finding electrons between the two nuclei;

* The distance between the cores has to allow the 1s orbitals to have the maximum overlap. For example, this value in the hydrogen molecule is around 0.74 angstrom. If this is not true, then there is talk of link length to define the distance that there are between two atoms covalently united;

* 52 kilocalories are necessary to cut the covalent bonds that exist in 1 gram of hydrogen gas.

With respect to covalent substances, it is possible to recognize the following two:

* the molecular covalent , that is to say the bonds that form molecules with low boiling and melting temperatures, insulators of heat and electric current, soluble in polar or apolar solvents (depending on whether polar or apolar are their own molecules ), such as benzene, nitrogen, oxygen and carbon;

* the reticular covalent , crystalline networks with an indefinite number of atoms, similar to ionic compounds, characterized by being very hard, insoluble and with high boiling and melting temperatures, such as diamond and quartz.

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