Explanation:

A double bond is a type of covalent bond in which two pairs of electrons are shared between two atoms. It consists of one sigma bond and one pi bond.

Sigma Bond:

- A sigma bond is formed when two atomic orbitals overlap head-on, resulting in the sharing of two electrons.
- It is a strong bond that allows free rotation of atoms around the bond axis.
- In a double bond, the first bond formed between two atoms is a sigma bond.

Pi Bond:

- A pi bond is formed when two p orbitals overlap sideways, resulting in the sharing of two electrons.
- It is a weaker bond than a sigma bond.
- In a double bond, the second bond formed between two atoms is a pi bond.

Double Bond:

- When two atoms form a double bond, they share a total of four electrons.
- The first bond formed is a sigma bond, which allows free rotation.
- The second bond formed is a pi bond, which restricts rotation due to overlapping sideways p orbitals.

Example:

Let's take the example of ethene (C2H4) to understand the concept of a double bond.

- In ethene, each carbon atom forms three sigma bonds and one pi bond.
- The sigma bonds are formed by overlapping sp3 hybrid orbitals of carbon atoms with 1s orbitals of hydrogen atoms.
- The pi bond is formed by overlapping unhybridized p orbitals of carbon atoms.

Therefore, a double bond is made up of one sigma bond and one pi bond. In the case of ethene, it has two sigma bonds and one pi bond, resulting in a double bond between the carbon atoms.

Conclusion:

In summary, a double bond consists of one sigma bond and one pi bond. The sigma bond is formed by head-on overlap of atomic orbitals, while the pi bond is formed by sideways overlap of p orbitals. The presence of a pi bond restricts rotation around the bond axis, making double bonds more rigid compared to single bonds.