chemistry questions

in 100 words or more each with references and in text citations answer the 2 questions.

1. The following are some of the limitations of the octet rule: The rule failed to predict the shape and relative stability of molecules. It is based upon the inert nature of noble gases. However, some noble gases like xenon and krypton form compounds such as XeF2, KrF2 etc.  I am thinking what could be some other limitations of the octet rule.  I will look forward to hear from you all on this.

2. What is a sigma bond? What is a pi bond? Must a double      bond have a sigma bond and a pi bond? Explain. A sigma bond is a covalent      bond formed due to the overlapping of orbitals of the two atoms along the      line joining the two nuclei (orbital axis) is called sigma (σ) bond. For      example, the bond formed due to s-s and s-p, p-p overlapping along the      orbital axis are sigma bonds. I am thinking what is a pi bond.  I      will look forward to hear from you all on this.