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I want to add to saal's point-- Carbon is _fantastic_. It easily forms bonds in 4 directions, but can instead form bonds in 3- or 2- or occassionaly 1- direction if need be. If you are choosing an element to be the "lego backbone" of a bunch of molecules, carbon is a great choice because it gives that versatility. Nitrogen can easily form 3 bonds, and Oxygen 2, and Boron 3.

So, you ask, what about Silicon? Silicon also has 4 valence electrons, so it ought to be a nice substitute, right?

The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room. So you still only have 4- connectors, but it's a larger element.

There's clearly an advantage into being able to form precise and small molecules. Carbon, with it's small size and 4 valence electrons, is a clear choice for a backbone.




> The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room

I'm a complete layman, but how does that play out in planets with gravity force significantly stronger or weaker than Earth? Woul that skew which elements make the best building blocks?


No, gravity for microscopic purposes may as well not exist. Surface tension matters for very small creatures.

Silicon is basically impossible as a building block for life. It forms bonds that are too strong, meaning it requires much more energy to fuel life processes and reducing the rate of chance collisions which lowers the prospects for abiogenesis.

Carbon is the only realistic choice; it is the only element abundant enough with the right balance of stability and versatility.


I guess I was thinking more about pressure than gravity (though the two are related). At extremely high or extremely low pressure, I would imagine things to play out differently


I think that in most situations the pressure would still not change much as far as atom-to-atom distances go. And in those situations, carbon is still smaller and workable.




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