We explain what a covalent bond is and some of its characteristics. Also, the types of covalent bonding and examples.
What is a covalent bond?
A type of bond is called covalent Chemical bond what happens when two atoms are linked to form a molecule, sharing electrons belonging to its valence layer or last energy level, thus reaching the well-known "stable octet", according to the "octet rule" proposed by Gilbert Newton Lewis on the electronic stability of atoms.
The "octet rule”States that ions of the chemical elements located in the Periodic table, tend to complete their last energy levels with 8 electrons, and this electronic configuration gives them great stability, which is very similar to that of electrons. Noble gases.
Covalently bonded atoms share one or more pairs of electrons from their last energy level. It is called molecular orbital to the region of space where the electron density is located in the molecule.
This electron density can be defined and calculated using very complex mathematical equations that describe the behavior of electrons in molecules. On the other hand, there are also atomic orbitals, which are defined as the region of space that represents the probability of finding an electron around the atomic nucleus. Thus, when several atomic orbitals are combined, molecular orbitals are generated.
Covalent bonds are formed by sharing electrons between the bonding atoms, and they differ from ionic bonds in which in the latter there is a transfer of electrons between the atoms involved in the ionic bond (no electrons are shared).
For an ionic bond to form, an atom transfers one or more electrons to another atom, and the bond is formed by electrostatic interaction between both atoms that are electrically charged, because when the transfer of electrons occurs, an atom (the one that gave electrons) it was left with a positive charge (cation) and the other atom (the one that accepted electrons) was left with a negative charge (anion).
On the other hand, the covalent bond is formed between atoms that do not have a large electronegativity difference. This bond can be formed between non-metallic atoms, or between metallic atoms and hydrogen. The ionic bond is formed between ions of atoms with a high electronegativity difference, and is usually formed between ions of atoms of metallic elements and ions of atoms of non-metallic elements.
It is important to clarify that there is no absolutely covalent bond, or an absolutely ionic bond. In fact, ionic bonding is often viewed as an "overstatement" of the covalent bond.
Covalent bond types
There are the following types of covalent bond, based on the number of electrons shared by the bonded atoms:
- Simple. Bonded atoms share one pair of electrons from their last electronic shell (one electron each). It is represented by a line in the molecular compound. For example: H-H (Hydrogen-Hydrogen), H-Cl (Hydrogen-Chlorine).
- Double. The bonded atoms each contribute two electrons from their last energy shell, forming a bond of two pairs of electrons. It is represented by two parallel lines, one above and one below, similar to the mathematical sign of equality. For example: O = O (Oxygen-Oxygen), O = C = O (Oxygen-Carbon-Oxygen).
- Triple. This bond is formed by three pairs of electrons, that is, each atom contributes 3 electrons from its last energy layer. It is represented by three parallel lines, located one above, one in the middle, and one below. For example: N≡N (Nitrogen-Nitrogen).
- Dative. A type of covalent bond in which only one of the two bonded atoms contributes two electrons and the other, however, none. It is represented by an arrow in the molecular compound. For example the ammonium ion:
On the other hand, according to the presence or not of polarity (property of some molecules to separate the electrical charges in their structure), it is possible to distinguish between polar covalent bonds (that form polar molecules) and nonpolar covalent bonds (that form nonpolar molecules). polar):
- Polar covalent bonds. Atoms of different elements and with electronegativity difference above 0.5. Thus, the molecule will have a negative charge density on the most electronegative atom, since this atom attracts the electrons of the bond with greater force, while a positive charge density will remain on the less electronegative atom. Separation of charge densities generates electromagnetic dipoles.
- Nonpolar covalent bonds. Atoms of the same element are bonded, or of different elements but with similar electronegativities, with an electronegativity difference of less than 0.4. The electron cloud is attracted with equal intensity by both nuclei and a molecular dipole does not form.
Examples of covalent bonding
Simple examples of covalent bonding are those that occur in the following molecules:
- Pure oxygen (O2). O = O (one double bond)
- Pure hydrogen (H2). H-H (a single link)
- Carbon dioxide (CO2). O = C = O (two double bonds)
- Water (H2O). H-O-H (two single bonds)
- Hydrochloric acid (HCl). H-Cl (a single bond)
- Pure nitrogen (N2). N≡N (a triple bond)
- Hydrocyanic acid (HCN). H-C≡N (one single and one triple bond)