why carbon compounds are stable and unreactive and of silicon are unstable and more reactive
Answers
First, I would not say that, because some are extremely reactive and unstable. However, carbon does form some bonds that are quite strong. The C - C bond is about 85 kcal/mol, which means long chains can be formed that do not break easily, and we end up with our hydrocarbons, or even diamond. Carbon also forms bonds to oxygen, hydrogen and nitrogen that are all in the range of about 100 kcal/mol. This has two effects. The first is, they are even harder to break than the C - C bond. The second is there is no great energy change in changing between them, which means you can make a number of compounds that do not automatically react with something else. This means that there is a vast number of possible compounds that can be made, can interchange through powerful catalysts, but do not change into something else easily on their own, which is the basis of life. This is the reason there will be no silicon life forms (other than created machines) - silicon bonds extremely strongly with oxygen, so most silicon compounds will spontaneously convert to rock! A final reason for great stability in carbon is that the eight-electron shell is extremely difficult to expand, which means that nucleophiles (compounds with lone pair of electrons) cannot attack it easily, and nucleophilic attack virtually requires another nucleophile to leave (thus hydrolysis reactions with carbon-halide bonds, excluding fluoride). In comparison, silicon can expand its valence shell, and permits water to attack some compounds very readily, thus making rock.