A covalent bond, also known as a molecular bond, is a bond formed between two atoms by mutual sharing of electrons. Typically, the sharing of electrons allows each atom to fill their outermost shell completely, and thus, helps them achieve their stable electronic configuration.
Question : Explain the formation of dichlorine (Cl2) molecule.
Answer : The atomic number of chlorine is 17 and it has 7 valence electrons (3s23p5). Chlorine needs one electron to attain stable electronic configuration of argon, which has 8 valence electrons — 3s23p6.
In Cl2 molecule, each chlorine atom contributes one electron to form a common shared paired. By doing so, both of them complete their octets. In other words, they acquire the electronic configuration of argon.
The shared pair of electrons can be represented by dash as given below:More Practice Questions
The number of electrons an atom shares with other atoms to form a chemical bond is called its covalency. For example, in Cl2 molecule, each chlorine atom shares one electron; therefore, the covalency of chlorine is 1. Similarly, in CH4 (methane), the covalency of carbon is 4, and that of hydrogen is 1.
Covalent Bond Between Different Types of Atoms
The formation of covalent bond does not necessarily require atoms of same element. Two atoms of different elements can also share electrons to form a covalent bond provided they are of comparable electronegativity. Example: In HCl molecule, hydrogen and chlorine contribute one electron each to form a shared pair of electrons.
Hydrogen and chlorine
Effect of Electronegativity on Covalent Bonds
When two atoms of same elements form a covalent bond, the electron pairs are shared equally between the two atoms. But, when a covalent bond is formed between atoms of different electronegativities, the shared pair of electrons gets displaced towards the atom which is more electronegative. As a result, one end of the molecule becomes slightly positively charged, and the other end becomes slightly negatively charged. For more, please read our polarity of bonds tutorial.
Types of Covalent Bonds
There are different types of covalent bonds:
Single, Double, and Triple Covalent Bonds
- Single covalent bond : In a single covalent bond, each atom contributes one electron to form a shared pair. A single bond is represented by a single dash (−).
- Double covalent bond : A covalent bond in which two pairs of electrons — contribution of two electrons by each atom — are shared between two atoms is called a double covalent bond. A double bond is represented by two dashes (=).
- Triple covalent bond : In a triple covalent bond, three pairs of electrons are shared between two atoms. A triple bond is represented by three dashes (≡).
Single bond in H2
Double bond in O2
Triple bond in N2
Polar and Non-polar Covalent Bonds
- Polar covalent bond : A polar covalent bond is formed when two atoms share electron pairs unequally. The situation arises when the bond is formed between atoms of different electronegativities. As a result, partially positive and negative charges are developed on the atoms — partially negative charge on more electronegative atom and partially positive charge on the other atom.
- Non-polar covalent bond : A non-polar covalent bond is formed when two atoms share electrons equally. The covalent bond between atoms of same elements such as in H2 and Cl2 is non-polar because both the atoms have equal electronegativity.
For more useful information, refer to polarity of bonds.
Sigma and Pi Bonds
When two atoms combine together to form a covalent bond, they are so close to each other that their orbitals are partially merged. This is called overlapping of orbitals.
There are two types of covalent bonds based on nature of overlap:
- Sigma (σ) bond
- Pi (π) bond
A single bond is usually a σ bond. A double bond consists of one σ bond and one π bond, and a triple bond consists of one σ and two π bonds. We will study about σ and π bonds in overlapping of orbitals.