Elements with very high ionization energies cannot transfer electrons, and elements with very low electron affinity cannot gain electrons. Atoms of such elements tend to share their electrons with atoms of other elements or with other atoms of the same element in such a way that both atoms acquire an octet configuration in their valence shell, thus providing stability. This type of relationship through the sharing of electron pairs between different or same species is known as Covalent Bond. Covalent Bonding Can Be Achieved In Two Ways:
1. Sharing of electrons between atoms of the same kind Ex. H 2 , Cl 2 , O 2, etc. formation .
2. Sharing of electrons between different types of atoms Eg. CH 4 , H 2 O, NH 3, etc. formation
Properties of Covalent Bond
If the normal valency of an atom is not met by sharing a single electron pair between atoms, atoms can share more than one electron pair between them. Some properties of covalent bonds are:
• Covalent bond does not cause the formation of new electrons. The bond just pairs them.
• They are very strong chemical bonds between atoms.
• A covalent bond normally contains about ~80 kilocalories per mole (kcal/mol) of energy.
• Covalent bonds rarely break spontaneously after forming.
• Covalent bonds are directional where the bonded atoms show certain orientations relative to each other.
• Most compounds with covalent bonds exhibit relatively low melting points and boiling points.
• Covalently bonded compounds generally have lower enthalpies of evaporation and fusion .
• Compounds formed by covalent bonds do not conduct electricity due to lack of free electrons.
• Covalent compounds are insoluble in water.
What is the Octet Rule?
All atoms except the 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 obtain stable electronic configurations. Therefore, the tendency of atoms of various elements to obtain the stable configuration of eight electrons in their valence shell is the reason for Chemical combination, and the principle of obtaining a maximum of eight electrons in the valence shell of atoms is called the octet rule.
Lewis introduced simple symbols to denote the electrons found in the outer shell of the atom, known as valence electrons. These symbols are known as Electron Dot Symbols and the structure of the compound is known as Lewis Dot Structure. The conditions for writing Lewis dot structures are as follows.
• Sharing an electron pair between atoms results in the formation of covalent bonds.
• During bond formation, each bond consists of two electrons contributed by each of the bonding atoms.
• By mutual sharing of electrons, each atom achieves an octal configuration in its valence shell.
Electron dot structures of covalent molecules are written according to the octet rule. According to this rule, all atoms in the molecule will have eight electrons in their valence shell except the Hydrogen atom. Hydrogen will only have two electrons because only two electrons complete its first shell to obtain the helium configuration.
Thus, elements of group 17 such as Cl will share an electron to obtain stable octet; Elements of group 16 such as O and S share two electrons; Elements of the 15th group share three electrons. For example, an oxygen atom with six electrons in its valence shell completes its octet by sharing two electrons with two hydrogen atoms, forming a water molecule.
Types of Covalent Bonds
Depending on the number of shared electron pairs, the covalent bond can be classified as:
• Single Covalent Bond
• Double Covalent Bond
• Triple Covalent Bond
Single Covalent Bond
A single bond is formed when only one electron pair is shared between the two atoms involved. It is denoted by a dash (-). This form of covalent bond is the most stable, although it has a smaller density and is weaker than double and triple bonds. For example, the molecule HCL has a Hydrogen atom with one valence electron and a Chlorine atom with seven valence electrons. In this case, a single bond is formed by sharing an electron between hydrogen and chlorine.
Double Covalent Bond
A double bond is formed when two pairs of electrons are shared between two participating atoms. It is represented by two dashes (=). Double covalent bonds are much stronger than a single bond, but they are less stable. For example, the Carbon dioxide molecule has a carbon atom with six valence electrons and two oxygen atoms with four valence electrons.
To complete its octet, carbon shares two of its valence electrons with an 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 its valence shell. Each atom needs two more electrons to complete its octets. Therefore, the atoms share two electrons each to form the oxygen molecule. Since the two electron pairs are shared, there is a double bond between the two oxygen atoms.
Ethylene molecule: Ethylene, each carbon atom shares two hydrogen atoms with two valence electrons with two other carbon atoms along with two electrons remaining. That is, there are double bonds between carbon atoms.
Triple Covalent Bond
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. For example: In the formation of a nitrogen molecule, each nitrogen atom with 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 is found where unequal sharing of electrons occurs due to the difference in electronegativity of the bonding atoms. The more electronegative atom will have a stronger attraction for electrons. The electronegative difference between atoms is greater than zero and less than 2.0. As a result, the shared electron pair will be closer to that atom.
For example, molecules that form hydrogen bonds as a result of an unstable electrostatic potential. In this case, the hydrogen atom interacts with electronegative fluorine, hydrogen or oxygen.
Non-Polar Covalent Bond
This type of covalent bond is formed when there is an equal share of electrons between the atoms. The electronegativity difference between the two atoms is zero. It occurs wherever bonding atoms have similar electron affinity (diatomaceous elements).
Example, Non-Polar Covalent Bond, Hydrogen gas, Nitrogen gas etc. found in gas molecules.
Polarization of Covalent Bonds
It is observed that in 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, a permanent dipole is formed in the bond and the covalent bond is said to be polarized. An example explaining the polarity of covalent bonds in a water molecule is given above. The more electronegative atom has 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 differ in their properties and structures. Covalent bonds involve pairs of electrons by two atoms connecting them in a fixed direction. The bond between two ions is called an ionic bond. Covalent bonds form between two non-metallic atoms, characterized by the sharing of electron pairs between atoms and other covalent bonds with an electronegativity difference greater than 2.0 (<2.0). In the case of covalent bond formation, polyatomic ions are formed. Whereas, ionic bond is formed as a result of electrostatic attraction between oppositely charged ions.
Difference Between Ionic and Covalent Bond
Covalent Bonds Ionic Bonds
A covalent bond forms between two similar electronegative nonmetals. This type of bond is formed between a metal and a nonmetal.
Covalent bonds have a specific shape. Ionic Bonds have no definite shape
Low Melting Point and Boiling Point High Melting Point and Boiling Point
Low Polarity and More Flammable High Polarity and Less Flammable
Covalent Bonds are Liquid or gaseous at room temperature At room temperature, Ionic Bonds are Solid.
Examples: Methane, Hydrochloric acid Example: Sodium chloride, Sulfuric Acid
The presence of bonding between two elements can be determined by calculating the electronegative value between the two atoms.
Bond Type electronegativity value
Polar Covalent Bond 0.5 to 1.9
Non-Polar Covalent Bond 0 to 0.4
Ionic bond 2.4 to 4.0
Writer: Can Baskin