State octet rule. Explain its inadequacies with respect to expanded octet.

Explain in brief with one example: Ionic bond.

Explain in brief with one example: Covalent bond.

Explain in brief with one example: co-ordinate bond

Give reason for: Sigma$\left(\mathrm{\sigma }\right)$ bond is stronger than pi$\left(\mathrm{\pi }\right)$ bond.

Give reason for: $\mathrm{HF}$ is a polar molecule.