Silicon is the element on the periodic table that is most similar to carbon. They are in the same “group” (column) of the periodic table, and just one “period” (row) below it. Silicon can create the same general kinds of chemical structures and bonds. It’s also abundant in the universe, so there should be no shortage of it.
There are, however, several reasons why carbon is a lot better. Carbon dioxide is a gas at most relevant temperatures and pressures. *Silicon* dioxide is also called *sand* and is a solid at any temperature that could make use of water. Likewise, “silane” and its longer cousins, which are analogous to methane, ethane, propane, etc., are highly flammable with oxygen and highly reactive with water, whereas methane and ethane only react with water under specific conditions. Silicon is heavier than carbon as well, which adds up.
You hear it mentioned because it’s loosely plausible and, when talking about alien life, it’s easy to forget that life *might* look very different elsewhere in the universe. But ultimately, silicon is unlikely because while you can get the right kinds of bonding, the chemistry is so completely different that it’s unlikely such things could form life.
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