The adjective covalent is used in the field of chemistry to describe the bond that is generated between atoms that have shared pairs of electrons. It is also qualified as covalent to that which has at least one covalent bond.
It is important to remember that particles that have an electrical charge and are made up of a non-neutral molecule or atom are called ions. Ions, according to the octet rule enunciated by the American Gilbert Newton Lewis in 1916, have a tendency to use eight electrons to complete the last energy levels and thus achieve stability in their configuration.
Atoms, to respect the octet rule, can use different kinds of chemical bonds to join. Among them appears the covalent bond, which supposes the sharing of electrons in the last level. This type of bond requires that the electronegativity difference between the atoms be less than 1.7.
According to abbreviationfinder.org, covalent bonds develop between atoms of different nonmetal elements 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 single, double or triple depending on the case. If the bond occurs between equal atoms that have an electronegativity difference of less than 0.4, a nonpolar covalent bond is obtained. On the other hand, if the bond is developed by atoms of different elements that have an electronegativity difference greater than 0.4, it is a polar covalent bond.
According to the chemists G. William Daub and S. Seese, in any covalent substance (such as a hydrogen molecule) the following four aspects are appreciated:
* Viewed singly, that is, outside of a combination, atoms have properties that are very different from those exhibited by molecules. For this reason, when writing the chemical formula of hydrogen, for example, we must put a two as a subscript of the H, since it is a diatomic molecule (one that is made up of two atoms, whether or not they are of the same chemical element) ;
* the two electrons are attracted to the two positive nuclei, something that happens with the aim of producing a more stable molecule than one in which the atoms are separated. This causes a covalent bond to be generated. Since the attraction to which the nuclei subject the electrons manages to cancel the repulsion between them, there is a good chance of finding electrons between the two nuclei;
* the distance between the nuclei has to allow the 1s orbitals to have the maximum overlap. For example, this value in the hydrogen molecule is around 0.74 angstroms. If this is not true, then we speak of bond length to define the distance between two covalently bonded atoms;
* 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:
* Molecular covalents, that is, bonds that form molecules with low boiling and melting temperatures, insulators of heat and electric current, soluble in polar or nonpolar solvents (depending on whether the molecules themselves are polar or nonpolar), such as benzene, nitrogen, oxygen and carbon;
* lattice covalents, crystal lattices 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.