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What is a covalent bond?

Covalent bonding is a strong interaction that allows two or more atoms to join together to form molecules. It is one of the types of chemical bonds in which a pair of electrons participates that comes from the outermost orbitals of the atoms that are bonded.

In theory, each atom contributes one of the two electrons at the moment of covalent bonding, in such a way that they gain stability. The pair of electrons, which is a kind of “electronic hook” between the two atomic nuclei, is shared equally between two identical atoms, but this is not the case when the two atoms are of two different elements.

There are atoms or elements that have a natural tendency to give up electrons, while others prefer to gain them.

For example, the famous water molecule, H 2 O, has two covalent bonds: HOH. Hydrogens and oxygen share the two pairs of electrons, but they orbit closer to the oxygen atom than to the hydrogen atom. This phenomenon is due to polarity and electronegativity.

Characteristics of covalent bonds

Covalent bonds have the following characteristics:

-They involve a pair of electrons for each covalent bond.

-Its lengths are usually expressed in units of pm (picometers) or Armstrong (Å).

-They are difficult to break, which means they are very strong.

-When they break, chemical changes or transformations occur, since the molecule is losing atoms. In this process energy is absorbed.

-When they are formed, a new molecule is born. In this process energy is released.

-Its lengths vary according to the avidity of one of the bonded atoms towards the pair of electrons; that is, the more it attracts it, the more the covalent bond lengthens.

-The simplest covalent bonds only exist between two atoms at a time, and are represented by a dash or dash (-).

There are other characteristics that apply to more complex covalent bonds, which however also conform to the above.

Types of covalent bonds

Covalent bonds can be classified, in principle, in two ways: according to the number of electrons that are shared, and according to the resulting polarity when the molecule is formed. Both will be briefly described below.

According to the number of shared electrons

Covalent bonds according to the number of shared electrons can be classified as single, double or triple. In the image above these respective bonds are shown for three molecules that will be discussed below.

  • Simple

The simple covalent bond is one in which there is only one pair of electrons, that is, two electrons. An example of this bond is seen in the water molecule, H 2 O. Note that of all the covalent bonds, the simple one, II, is the longest of all. This is almost always the case.

  • Double

In the double covalent bond we have, as the name suggests, two pairs of electrons, which means that there are four electrons that reinforce the “electronic hook” between the two bonded atoms.

That is, a double bond (=) is stronger than a simple one (-). However, it is more reactive, which although it may seem contradictory, is easy to explain considering the overlaps of the atomic orbitals.

An example of this bond is seen in the carbon dioxide molecule, CO 2 . The two oxygen atoms remain strongly bound to carbon thanks to this bond, O = C = O, which, being stronger, brings the atoms closer together; and in turn this leads to the link being shortened.

  • Triple

The triple bond is the strongest and shortest of the covalent (common) bonds. Note that for the nitrogen molecule, N 2 or N≡N, its atoms represented by bluish spheres are very close to each other.

In the triple bond we have three pairs of electrons, which is equal to six electrons that reinforce the covalent bond between the two atoms.

  • Dative

The dative bond is a special type of single bond that consists of the pair of electrons coming from only one of the two bonded atoms. That is, it could be seen as a “forced sharing”.

For example, the H 2 O molecule uses one of the free oxygen pairs to donate to an H + ion (H 2 O: → H + = [H 2 O-H] + or H 3 O + ).

According to polarity

Covalent bonds according to polarity can be classified as polar covalent, or nonpolar covalent.

  • Polar covalent bonds

It occurs between two different atoms whose electronegativity difference is large. This results in the formation of a dipole, where one atom is negatively concentrated (attracts electrons), while the other is positively concentrated (gives up electrons). For example, the covalent bond of HF (HF) is polar, since fluorine is more electronegative than hydrogen.

  • Nonpolar covalent bonds

It occurs between two identical atoms whose electronegativity difference is zero or very small. The molecules mentioned above: I 2 , O 2 and N 2 have nonpolar covalent bonds. Therefore, the electrons hover near both atoms with the same frequency.

Examples of covalent bonds

Finally, other covalent bonds will be listed in addition to those already explained, also indicating their types:

H2S (MSM), polar simple covalent

HCl (H-Cl), polar single covalent bond

H2 (HH), simple covalent nonpolar

C2H4 (HC≡CH), with two polar single bonds (CH) and one non-polar triple bond (C≡C)

F2 (FF), simple covalent nonpolar

P2 (P≡P), nonpolar (or apolar) double covalent bond

CO₂ (O = C = O), double polar covalent

S2 (S = S), nonpolar double covalent bond

HCN (HC≡N), with a single polar covalent bond (HC), and another polar triple bond (C≡N)

NH3 (NH), with three polar simple covalent bonds

The cited examples correspond to small molecules. But covalent bonds are present in all existing molecules, including polymers, biomolecules , macromolecules, minerals, ceramics, hydrocarbons, among many others.

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