Covalent Bond: Definition & Types

What Is a Covalent Bond?

A covalent bond is the bond formed when two atoms share their electrons equally. The pair of electrons that form this bond is called a bonding pair or shared pair. Covalent bonds are known as molecular bonds. The bonding pairs ensure the stability of their outer shell by achieving 8 total electrons on this shell. 

Covalent bond definition

Atoms with high ionisation energies are unable to transfer their electrons while those with low ionization energy cannot receive electrons. So, these atoms tend to equally share their electrons with other elements or those of the same element, so that both of them can obtain their octet configuration in their valence shell. This stabilizes the atom. This kind of association between atoms of the same element or different elements is called Covalent Bond.

Electron sharing between atoms of the same element- Formation of H2, Cl2, O2.

Sharing of electrons between different elements – CH4, H2O, NH3.

Types of Bonding

There are four main types of bondings called covalent bonding, hydrogen bonding, van der Waals bonding, and ionic bonding. 

• Covalent bonding is the sharing of electrons between any two atoms.
• Hydrogen bonding is a partial energy bonding between hydrogen and oxygen molecules where one atom gains a slight positive charge while the other receives a negative charge. 
• Van der Waals force attracts neutral molecules to each other due to their slight polar charges. 
• Ionic bonds: Here two ions of opposite charges are attracted to each other and share their electrons. 

Covalent Bonding in Carbon Atom

According to its electronic configuration Carbon either needs to acquire or lose 4 electrons to achieve stability. Since it cannot gain electrons and become a negative ion since that will affect the proton-electron balance, or lose 4 electrons as it will not be good for its energy balance to lose those electrons later. Both scenarios will affect the stability of the atoms.

The only solution here is to neither gain nor lose the electrons, instead share the necessary number of electrons and form a covalent bond. 

Properties of Covalent Bond

In a covalent bond, atoms share more than a single pair of electrons to help them attain stability. Some of the properties of covalent bonds are:

• Covalent bonding does not lead to new electron formation.
• These are some of the most powerful bonds between two atoms.
• The energy of a covalent bond is usually ~80 kcal/mol.
• They don’t succumb to spontaneous breakage of the bonds. 
• Atoms in the covalent bonds show specific orientations to one another.
• Compounds with covalent bonds have lower melting points, boiling points, lower enthalpies of fusion and vaporization. 
• They are not good conductors of electricity. 
• These compounds are insoluble in water.

What is the Octet Rule?

Atoms having 8 electrons in their outer shell attains stability. Eg. Noble gases. It means that all other atoms do not have stability on their own. So they participate in bonding with other atoms to fulfil the 8 electrons in their valence shells. 

This tendency of atoms to gain stability by having 8 electrons in their outer shells is called a Chemical combination. This principle that enables them to gain stability with 8 electrons is called the Octet Rule. 

Types of Covalent Bonds

Depending on the number of electron pairs, the covalent bond is divided into three types. 

• Single Covalent Bond
• Double Covalent Bond
• Triple Covalent Bond

Single Bonds

Here, only one pair of electrons are shared between the atoms. This bond is denoted with a (-) between the said atoms. It is a stable configuration but weaker than the other two types. 

For example, the HCl molecule has one Hydrogen atom that has one valence electron and one Chlorine atom has 7 valence electrons. Here the single bond formed is by sharing one electron from each atom. 

Double Bonds

A double bond will have two pairs of electron sharing between the involved atoms. The double bond is represented as (=). It is stronger than a single bond but less stable.

Example 1: Carbon dioxide. Here the carbon atom shares two electrons with each oxygen atom, while each oxygen atom shares two electrons with the carbon atom. Here the carbon atom forms two double bonds with each of the oxygen molecules. 

O=C=O

Example 2, Ethylene Molecule: In ethylene (C2H4), each carbon atom will be sharing two electrons and one each with two hydrogen atoms. Here, the double bond is between the carbon atoms. 

Triple Bond

When three pairs of electrons are shared between atoms, it forms a triple bond. Such bonds are written as three dashes (≡). These are the least stable form of covalent bonds. 

In a nitrogen molecule (N2), each nitrogen atom shares three electrons forming a triple bond. 

Polar Covalent Bond

A polar covalent bond is formed when the sharing of electrons is unequal due to the difference in their electronegativity. Here one atom will have a stronger pull on the electrons. The difference in their electronegativity should be 2-2.0. Here the atom with stronger electronegativity will have the paired electrons towards it than the other atom. 

Nonpolar Covalent Bond

This type of covalent bond is formed when the bonding atoms have the same electronegativity. Their net electronegativity will always be zero. For example, gaseous molecules such as Hydrogen, Nitrogen, etc.

Polarization of Covalent Bonds

It is observed that in the polar bonds, the electron cloud moves closer to the higher electronegative atom. This causes a permanent dipole between the bonds. Such covalent bonds are said to be polarized. Here the higher electronegative atom will have a negative charge while the other one will be positively charged.