Introduction

A molecule is formed when the electrons of two or more atoms are transferred or shared among the combining atoms. This leads to the formation of the bond among those atoms that keeps them associated with each other. The nature of the bond formed depends on the type of electron transfer activity. 

To form molecules, atoms don’t transfer electrons each time as there are various restrictions for electron transfer; hence, electron sharing is a better option in that case. It forms a covalent bond. There are various types of covalent bonds depending on the number of electrons shared, and they have their unique representation that is provided in this covalent bond study material.

Stability of covalent bond – octet rule

The major question before scientists was the reason for covalent bond formation. There were different types of bonds and different numbers of electron sharing for bond formation. It was noted that atoms share electrons so that they can achieve their nearest noble gas configuration. 

In this way, each combining atom has 8 electrons in its valence or the outermost shell. This provides high stability to the combining atoms, and the molecule is highly stable than the combining atoms as the covalent bonds have high strength.

Though there are some cases in which a duplet is formed instead of the octet, i.e., the outermost shell has 2 electrons. This happens when,

• The combining atoms have less number of electrons.
• The nearest noble gas configuration is that of Helium.
• Hydrogen generally forms a duplet as it has only 1 electron for sharing.

This covalent bond study material covers the types of covalent bonds, factors affecting their formation, etc., and this all is based on the stability and completion of the octet, known as the octet rule.

Formation of covalent bonds

In some cases, one atom might be more electronegative than the other combining atom. Then, the more electronegative atom will try to attract the shared pair of electrons towards itself and develop a partial negative charge. The charge developed on the more electronegative atom is not permanent and is weak, but it gives rise to some polar nature in the bond. This happens when one of the combining atoms is fluorine, oxygen, nitrogen, or any other highly electronegative atom.

Factors affecting the formation of covalent bond

Generally, atoms try to combine by electron transfer as that provides high strength to their bonds as the force of attraction is very strong in them. 

Due to various factors, electron transfer is not possible in every case; that’s why atoms share electrons to complete their octet and gain stability. These factors are based on the type of atoms and their properties.

• The atomic size plays a crucial role in determining the strength of the covalent bonds as the electrons are shared to form it. If the size of combining atoms is large, then the shared pair of electrons will be farther from the nucleus and experience less force of attraction. This will result in weak covalent bond formation. Hence, small atomic radii form strong covalent bonds.
• There should be a balance between ionisation enthalpy and electronegativity because the atoms should be willing to give and receive electrons for the mutual electron sharing process. Hence, high ionisation enthalpy and low electronegativity are preferred for strong covalent bonds formation.
• Electron affinity should be high as it determines the stability of bond formation. High energy bonds are less stable; hence, the release of energy makes the covalent bonds stable, and this is the ultimate goal of bond formation.

Properties of covalent bond

There are certain specific properties in terms of energy, chemical nature, and strength associated with the covalent bonds. This helps in the study of their structure and behaviour in different environments to get a deep insight into organic and inorganic chemistry. 

• Covalent bonds are both polar and non polar as they are formed by mutual sharing of electrons that should have the required amount of ionisation enthalpy and electronegativity, e.g., SF6, CO2, etc. but there are some covalent bonds that are polar due to hydrogen bonding, e.g., HF, H2O, etc.
• Though they have high strength, their melting and boiling points are low as compared to ionic bonds, as the ionic bonds have high lattice enthalpy. Due to this, many covalent bond molecules are highly volatile.

• During the formation of covalent bonds, there is no decrease in the total number of electrons in any combining atoms. The number of electrons for any combining atoms either increases or remains the same as there is a mutual sharing of electrons.
• As electrons are shared, they don’t have any free electrons; hence, they are bad conductors of electricity. Also, they are not water-soluble.
• Covalent bonds are formed by electron sharing in a specific orientation giving a unique shape to the molecule. 

These features determine the physical and chemical properties of the molecules formed by covalent bonds; hence, their study is important.

Representation of covalent bond: Lewis dot structures

Covalent bonds are formed by sharing electrons that are present in the outer shell. To represent these electrons, dot symbols are used known as electron-dot symbols. In the lewis dot structure,

• Each bond has two electrons contributed by the two combining atoms, where both atoms contribute an equal number of electrons.
• The remaining electrons of the outer shell are represented by dot structures.
• For example, H:H, in this, each hydrogen atom contributes a single electron, and the shared pair has two electrons that belong to both the atoms. In this way, the duplet of hydrogen is formed.

Different types and representations of covalent bond

Based on the number of electrons available for sharing to form covalent bonds, there are three types of covalent bonds.

• Single Covalent Bonds: In this type, each combining atom contributes one electron for bond formation, e.g., H-H

Here, a single bond is represented by a single dash.

• Double Covalent Bonds: In this, each combining atom contributes two electrons for sharing, e.g., H₂C=CH₂. Here, the carbon-carbon bond is a double bond and is represented by two straight lines.
• Triple Covalent Bonds: In this bond, each combining atom contributes three electrons to form a bond, e.g., HC≡CH
  Here, the carbon-carbon bond is represented by three straight lines.

Conclusion

In this study material, the formation of covalent bonds and the specific criteria for their formation are described. This study helps in determining the properties of a particular molecule and its application in the industry to make a particular tool or compound with desired physical and chemical properties. This helps in determining the formation of oxides, sulphides, etc., depending on the nature and strength of the bonds formed, and their reactivity in a particular environment. This also helps in the determination of the magnetic and electric properties of a molecule on the application of magnetic and electric fields of specific strength.