A covalent bond is formed by equal sharing of electrons from both the participating atoms. The pair of electrons participating in this type of bonding is called shared pair or bonding pair. The covalent bonds are also termed as molecular bonds. Sharing of bonding pairs will ensure that the atoms achieve stability in their outer shell which is similar to the atoms of noble gases.
Table of Content
- Covalent Bonding in Carbon
- Octet Rule
- Types of Covalent Bond
- Polarization of Covalent Bond
- Covalent vs Ionic Bonds
- Solved Examples
What is Covalent Bond?
Elements having very high ionisation energies are incapable of transferring electrons and elements having very low electron affinity cannot take up electrons. The atoms of such elements tend to share their electrons with the atoms of other elements or with other atoms of the same element in a way that both the atoms obtain octet configuration in their respective valence shell and thus achieve stability. Such association through sharing of electron pairs among different or same kinds is known as Covalent Bond.
Covalent Bonding can be Achieved in two Ways:
- Sharing of electrons between atoms of the same kind E.g. Formation of H2, Cl2, O2, etc.
- Sharing of electrons between atoms of different kind E.g. Formation of CH4, H2O, NH3, etc.
Covalent Bonding in Carbon Atom
As per the electronic configuration of Carbon, it needs to gain or lose 4 electrons to become stable, which seems impossible as:
- Carbon cannot gain 4 electrons to become C4-, because it will be tough for 6 protons to hold 10 electrons and so the atom will become unstable.
- Carbon cannot lose 4 electrons to become C4+ because it would require a large amount of energy to remove out 4 electrons and also the C4+ would have only 2 electrons held by proton, which will again become unstable
Carbon cannot gain or donate electrons, so to complete its nearest noble gas configuration, it shares electron to form a covalent bond.
Properties of Covalent Bond
If the normal valence of an atom is not satisfied by sharing a single electron pair between atoms, the atoms may share more than one electron pair between them. Some of the properties of covalent bonds are:
- Covalent bonding does not result in the formation of new electrons. The bond only pairs them.
- They are very powerful chemical bonds that exist 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.
- Covalent bonds are directional where the atoms that are bonded showcase specific orientations relative to one another.
- Most compounds having covalent bonds exhibit 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.
What is the Octet Rule?
All atoms except noble gases have less than eight electrons in their valence shell. In other words, the valence shells of these atoms do not have stable configurations. Therefore, they combine with each other or with other atoms to attain stable electronic configurations.
“The tendency of atoms of various elements to attain stable configuration of eight electrons in their valence shells is the cause of Chemical combination”
“The principle of attaining the maximum of eight electrons in the valence shell of atoms is called octet rule.”
Lewis introduced simple symbols to denote the electrons present in the outer shell of atom known as the valence electrons. These symbols are known as Electron Dot Symbols and the structure of the compound is known as Lewis Dot Structure.
Conditions for writing the Lewis dot structures
- Sharing of an electron pair between the atoms results in the formation of covalent bonds.
- During bond formation, each bond consists of two electrons which are contributed by each one of the combining atoms.
- By the mutual sharing of electrons, each atom attains octet configuration in its valence shell.
Electron dot structures of covalent molecules are written with respect to the octet rule. According to this rule, all the atoms in the molecule will have eight electrons in their valence shell except the Hydrogen atom. Hydrogen will have only two electrons because only two electrons complete its first shell to attain helium configuration.
Thus the elements of group 17 such as Cl would share one electron to attain stable octet; the elements of group 16 such as O and S would share two electrons; the elements of group 15 would share three electrons and so on.
For Example, the oxygen atom which has six electrons in its valence shell completes its octet by sharing its two electrons with two hydrogen atoms to form a water molecule.
Types of Covalent Bond
Depending upon the number of shared electron pairs, the covalent bond can be classified into:
- Single Covalent Bond
- Double Covalent Bond
- Triple Covalent Bond
A single bond is formed when only one pair of the electron is shared between the two participating atoms. It is represented by one dash (-). Although this form of covalent bond has a smaller density and is weaker than a double and triple bond, it is the most stable.
For 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.
A double bond is formed when two pairs of electrons are shared between the two participating atoms. 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 atom with four valence electrons.
To complete its octet, carbon 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 CO2.
Oxygen-Molecule: In the formation of the oxygen molecule, each oxygen atom has six electrons in their valence shell. Each atom requires two more electrons to complete their octet. Therefore the atoms share two electrons each to form the oxygen molecule. Since two electron pairs are shared there is a double bond between the two oxygen atoms.
Ethylene Molecule: In ethylene, each carbon atom shares two of its valence electron with two hydrogen atoms and remaining two electrons with the other carbon atom. So there is a double bond between the carbon atoms.
A triple bond is formed when three pairs of electrons are shared between the two participating atoms. Triple covalent bonds are represented by three dashes (≡) and are the least stable types of covalent bonds.
In the formation of a nitrogen molecule, each nitrogen atoms having five valence electrons provides three electrons to form three electron pairs for sharing. Thus, a triple bond is formed between the two nitrogen atoms.
Polar Covalent Bond
This type of covalent bond exists where the unequal sharing of electrons occurs due to the difference in the electronegativity of combining atoms. More electronegative atom will have a stronger pull for electrons. As a result, the shared pair of electrons will be closer to that atom.
Example, molecules forming hydrogen bonding as a result of an unbalanced electrostatic potential. In this case, the hydrogen atom interacts with electronegative fluorine, hydrogen, or oxygen.
Nonpolar Covalent Bond
This type of covalent bond is formed whenever there is an equal share of electrons between atoms. It occurs wherever the combining atoms have similar electron affinity (diatomic elements).
Polarization of Covalent Bonds
It is observed that in the sigma bonds between two different atoms, the electron cloud is always closer to the more electronegative of the two atoms participating in the sigma bond. Due to this, there is a permanent dipole that arises in the bond and the covalent bond is said to be polarized.
An illustration describing the polarity of the covalent bonds in a water molecule is provided above. The more electronegative atom is said to have a partial negative charge and the less electronegative atom has a partial positive charge in the polar covalent bond.
Difference Between Covalent and Ionic Bonds
Covalent bonds and ionic bonds are types of atomic bonds. These bonds are different in their properties and structure. The covalent bonds include pairs of electrons by two atoms binding them in a fixed orientation. While a bond between two ions is called ionic bonds.
Covalent bonding occurs between two non-metallic atoms characterized by the sharing of electron pairs between the atoms and other covalent bonds whereas, the ionic bond is formed as a result of electrostatic attraction between the oppositely charged ions.
Ionic Bond vs Covalent Bond
Difference Between Ionic and Covalent Bond
|A covalent bond is formed between two similar electronegative non-metals||This type of bond is formed between a metal and non-metal|
|Bonds formed from covalent bonding have a Definite shape||Ionic Bonds have No definite shape|
|Low Melting Point||High Melting Point|
|Low Polarity||High Polarity|
|Covalent Bonds are in Liquid or gaseous State at room temperature||At room temperature, Ionic Bonds have Solid state.|
|Examples: Methane, Hydrochloric acid||Example: Sodium chloride, Sulfuric Acid|
Draw the Lewis Structure of:
- Carbon Tetrachloride (CCl4)
- Ammonia (NH3)
1. Carbon has four electrons in its valence shell completes its octet by sharing its four electrons with four chlorine atoms to form carbon tetrachloride (CCl4) molecule as shown below.
2. Nitrogen has five electrons in its valence shell completes its octet by sharing its three electrons with three hydrogen atoms to form NH3 (Ammonia).