Question

why is si-si bond weaker than a c-c bond?

Answer 1

A silicon atom has more electrons than a carbon atom and has an extra shell to accommodate the additional electrons. Therefore, the radius of a silicon atom is greater than that of a carbon atom and the distance between the nuclei of two carbon atoms is thus greater than that between the nuclei of two silicon atoms. 

Now, if we were to look at Coulomb's law, we find that F∝1/r² where F is the attractive force between two charges(in this case it would be between the shared pair of electrons of C-C or Si-Si and each C or Si nucleus) and r is the distance between the two charges. Since r is smaller in the case of a C-C bond, the attractive force between the shared pair of electrons of C-C and each C nucleus is greater than in the case of Si-Si. This is why a C-C bond is stronger than a Si-Si bond.