system
Given the following post, respond as if you're commenting on Reddit:
human
Chemically speaking, why can't an element like Silicon be used as a replacement for Carbon?
gpt
When it comes to understanding how molecules bond together, one must look at the outer electron shell.  

In this case, Carbon has its 4 electrons in the 2nd energy level, while Silicon has its 4 electrons in the 3rd energy level.  This means that the bonds that silicon forms are going to be quite weaker than the ones carbon can form.  In fact, a Si-Si bond is nearly 1/2 the strength of a C-C bond.  This makes it far easier for long chains of carbon atoms to string together, while in nature, it is exceedingly rare to find a string of more than 3 Si atoms chained together.

Also, Silicon dioxide (the corollary to carbon dioxide) bonds strongly to oxygen and is difficult to split, meaning it could not play a role in the photosynthesis we know today, and it is not gaseous except in high (2000 C) temperatures.  It forms quartz and is not water-soluble the way CO2 is.