Covalent Bond

What is a Covalent Bond?

In Chemistry, covalent bonds are formed between two atoms or ions in which the electron pairs are shared between them; they are also known as molecular bonds. The forces of attraction or repulsion between two atoms (when they share an electron pair or bonding pair) are called Covalent Bonding. To describe the number of electron pairs shared by neighbouring atoms, Irving Langmuir introduced the term "covalence" in 1919 but it came into use in 1939. Compounds that contain carbon exhibit this type of chemical bonding. The pair of electrons which are shared by the two atoms now extend around the nuclei of atoms leading to the creation of a molecule.


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Covalent Bond Properties

If the valence of an atom is not satisfied by sharing a single electron pair between atoms, the atoms share more than one electron pair between them. Some of the covalent bond properties  are:

  • Covalent bonds are formed between non-metallic elements like hydrogen, oxygen, etc.

  • Covalent bonding does not result in the formation of new electrons. The bond only pairs them.

  • Covalent bonds include single, double, or triple bonds where 2, 4, or 6 electrons are shared respectively. 

  • There exist very powerful chemical bonds between atoms.

  • A covalent bond normally contains the energy of about ~80 kilocalories per mole (kcal/mol).

  • Covalent bonds rarely break spontaneously after it is formed.

  • Most compounds with covalent bonds have relatively low melting points and boiling points.

  • Compounds with covalent bonds usually have lower enthalpies of vaporization and fusion.

  • Compounds formed by covalent bonding don’t conduct electricity due to the lack of free electrons.

  • Covalent compounds are not soluble in water.

Types of Covalent Bonds

The covalent bond can be classified into three types depending upon the number of shared electron pairs. Types of covalent bonds are:

  • Single Covalent Bond

  • Double Covalent Bond

  • Triple Covalent Bond

Single Covalent Bond

When only one pair of the electron is shared between the two participating atoms then such bonds are said to be single covalent bonds. It is represented by one dash (-). This form of covalent bond has a smaller density and is weaker than a double and triple bond though it is the most stable bond.


Example: 

HCL molecule has one Hydrogen atom with one valence electron and one Chlorine atom with seven valence electrons. In this case, a single bond is formed between hydrogen and chlorine by sharing one electron thus completing its octet of one molecule of HCL.


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Double Bonds

When two pairs of electrons are shared between the two participating atoms a double bond is formed. It is represented by two dashes (=). Double covalent bonds are much stronger than a single bond, but they are less stable.


Example: 

Carbon dioxide molecule has one carbon atom with six valence electrons and two oxygen atoms with four valence electrons.

To complete its octet, as carbon has 6 valence electrons it shares two of its valence electrons with one oxygen atom and two with another oxygen atom. Each oxygen atom shares its two electrons with carbon and therefore there are two double bonds in a molecule of CO2.

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Triple Bond

When three pairs of electrons are shared between the two participating atoms a triple bond is formed. Triple covalent bonds are represented by three dashes (≡). These are the least stable types of covalent bonds.


Example:

In the formation of a nitrogen molecule, each nitrogen atom having five valence electrons provides three electrons to each other to form three electron pairs for completing the octet. Therefore, a triple bond is formed between the two nitrogen atoms.

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Depending on the affinity for the electrons that each atom has, we have three types of bonds polar, nonpolar and coordinated.


Nonpolar Covalent Bond

This union is established between atoms with equal electronegativity. This type of bond can also be maintained between atoms with an electronegativity difference of less than 0.4.


Example:

The chlorine molecule Cl2 is made up of two chlorine atoms with the same electronegativity, which share an electron pair in a nonpolar covalent bond. The same happens in the case of the two oxygen atoms to form the oxygen molecule O2.

Between the carbon atoms in organic molecules, the covalent bond is of the nonpolar type.


Polar Covalent Bond

The polar covalent bond is formed between two non-metallic atoms that have an electronegativity difference between 0.4 and 1.7. When these interact, the shared electrons stay closer to that more electronegative atom.


Example:

The water molecule has two polar covalent bonds between oxygen and hydrogens.

In the water molecule H2O, the electrons of the hydrogens stay closer and longer around the oxygen, which is more electronegative.

Fluorine F is the most electronegative element (4.0) and has seven valence electrons. When combined with hydrogen, hydrogen fluoride HF is formed, via a polar covalent bond.

The NH3 ammonia molecule has polar covalent bonds between nitrogen and hydrogens.


Coordinated or Dative Covalent Bond

This type of bond occurs when one of the atoms in the bond is the one that provides the electrons to share. We achieve this in the reaction between NH3 ammonia and boron trifluoride BF3. Nitrogen has two free electrons and boron is electron deficient. By combining both nitrogen and boron they complete their last shell with eight electrons.

FAQ (Frequently Asked Questions)

1. What is the Octet Rule?

Answer: All atoms except noble gases have less than eight electrons in their valence shell. Atoms transfer, accept or share electrons to complete their valence level with eight electrons. This is because atoms seek their most stable electronic configuration. Therefore, the tendency of atoms of various elements to attain a stable configuration of eight electrons in their valence shells is the cause of chemical combination. And the principle of attaining the maximum of eight electrons in the valence shell of atoms is called octet rule.

Beryllium and Boron are examples where the octet rule is not followed.

2. What are Types of Chemical Bonds.

Answer: In chemical bonding when a substance yield compounds, the stability of the resulting compound can be analysed by the type of chemical bonds it contains.


These types of bonds in chemical bonding are formed from the loss, gain, or sharing of electrons between two atoms or molecules; they also vary in strength and properties. There are 4 basic types of chemical bonds which are formed by atoms or molecules to yield compounds. These types of chemical bonds include:

  • Ionic Bonds

  • Covalent Bonds

  • Hydrogen Bonds

  • Polar Bonds