Describe the difference between sigma and pi bonds.

Sigma bonds are single covalent bonds, while pi bonds are additional bonds that occur in double or triple bonds.

Sigma (σ) and pi (π) bonds are types of covalent bonds that differ in the orbital overlap. A sigma bond is the strongest type of covalent bond, in which the atomic orbitals directly overlap between the nuclei of two atoms. They are formed by end-to-end overlapping and can occur between any kind of atomic orbitals; s-s, s-p, and p-p. Sigma bonds allow for free rotation, which means the atoms on either side of the bond can rotate without breaking the bond.

On the other hand, pi bonds are weaker than sigma bonds. They are formed by the lateral overlap of two p-orbitals, creating two regions of bonding. Pi bonds do not allow for free rotation. This is because the overlapping p orbitals would have to break apart, disrupting the electron cloud, before they could rotate. This makes molecules with pi bonds, such as alkenes and alkynes, more rigid in their structure.

In terms of electron distribution, in a sigma bond, the electron density is concentrated symmetrically along the axis of the bond, making it cylindrically symmetrical. In contrast, in a pi bond, the electron density is concentrated above and below the plane of the nuclei of the two bonded atoms.

It's also important to note that a single bond is always a sigma bond, while double bonds consist of one sigma and one pi bond, and triple bonds consist of one sigma and two pi bonds. This is because a single orbital can only form one sigma bond, while it can form up to two pi bonds. Therefore, multiple bonds (double and triple) must contain pi bonds.

In summary, sigma and pi bonds differ in their strength, rotation ability, formation, and electron distribution. Understanding these differences is crucial for understanding the properties and behaviours of different molecules.