Re: Alternate basis for life
From a chemistry perspective, the viability of life based on other elements comes down to what bonds the elements involved can form, and their relative energies.
Carbon is such a suitable element for life because of the range of different bonding structures it can form with other abundant elements, particulalrly with hydrogen and itself. Carbon can form chains and cyclic compounds because the bond energy of the C-C bond is not too far from that of C-H bonds, meaning that not too much energy is required to shuffle the configurations around. C-O and C-S bonds are in a lower energy range (energetically favourable, hence the ability to burn carbon compounds).
Contrast silicon, the next element down the periodic table from Carbon, and the one most often bandied around when people talk about alterntive chemistries for life. Silicon, like carbon has four electrons available for bonding, so like carbon, can form a myriad of structures. The bond energies are, however, less favourable. Si-O bonds are of a significantly lower energy than Si-Si and Si-H bonds (thus more favoured, forming the bond releases energy). Silicon dioxide, unlike carbon dioxide, is a crystalline solid. Once in this state, it is very difficult for life to do anything with it. Unlike carbon, the range and variety of structures that the element can make is largely dominated by such crystalline solids.
Whilst silicon can form chains, cyclic compounds, and liquids, these are predominantly compounds of alternating silicon and oxygen atoms. These are too stable to be easily broken down and therefore cannot store energy in the way a C-C bond can. Contrast silicone oil, and petroleum - one is virtually inert, whilst the other is a fuel source. Life needs to be able to shuffle energy about, store it, and release it on tap. Carbon is just too well suited to these roles for other elements to do a comparable job.