We explain what a covalent bond is and some of its characteristics. Also, the types of covalent bond and examples.
What is a covalent bond?
It is called a covalent bond a type of chemical bond that occurs when two atoms bond to form a molecule sharing electrons belonging to its valence shell or last energy level, thereby reaching the well-known “stable octet”, in accordance with the “octet rule” proposed by Gilbert Newton Lewis on the electronic stability of atoms.
The “octet rule” states that the 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 the noble gases.
Atoms linked by covalent bonds 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 They are formed by the sharing of electrons between bonding atoms and they differ from ionic bonds in that in the latter a transfer of electrons occurs between the atoms involved in the ionic bond (electrons are not 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 become electrically charged, since when the transfer of electrons occurs, an atom (the one that gave up electrons) 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 It is formed between atoms that do not have a large difference in electronegativity. This bond can form between non-metal atoms, or between metal atoms and hydrogen. The ionic bond is formed between ions of atoms with a high difference in electronegativity, 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, the ionic bond is often considered an “exaggeration” of the covalent bond.
Types of covalent bond
There are the following types of covalent bond, based on the number of electrons shared by the bonded atoms:
- Simple Bonded atoms share a pair of electrons from their last electron shell (one electron each). It is represented by a line in the molecular compound. For example: HH (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 equality sign. 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 shell. It is represented by three parallel lines, located one above, one in the middle and the other 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, depending on the presence or absence of polarity (property of some molecules of separating electric charges in their structure), we can distinguish between polar covalent bonds (which form polar molecules) and non-polar covalent bonds (which form non-polar molecules). polar):
- Polar covalent bonds Atoms of different elements are linked and with a difference in electronegativity above 0.5. Thus, the molecule will have a negative charge density on the most electronegative atom, since this atom attracts the bond electrons more strongly, while a positive charge density will remain on the less electronegative atom. The separation of charge densities generates electromagnetic dipoles.
- Nonpolar covalent bonds. Atoms of the same element, or of different elements but with similar electronegativities, are linked, with a difference in electronegativity less than 0.4. The electron cloud is attracted with equal intensity by both nuclei and a molecular dipole is not formed.
Examples of covalent bond
Simple examples of covalent bond are those that occur in the following molecules:
- Pure oxygen (O2). O=O (a double bond)
- Pure hydrogen (H2). HH (a simple link)
- Carbon dioxide (CO2) O=C=O (two double bonds)
- Water (H2EITHER) HOH (two single bonds)
- Hydrochloric acid (HCl) H-Cl (a single bond)
- Pure nitrogen (N2) N≡N (a triple bond)
- Hydrocyanic acid (HCN) HC≡N (one single bond and one triple bond)
