It’s often suggested that the aliens from the Aliens movies, sometimes referred to as xenomorphs, are silicon based rather than carbon based like us. There are a lot of silicon based aliens in science fiction, but no one knows if such a thing is really possible.
Carbon and silicon have one thing in common: they both have four bonding sites, meaning they can bond with up to four other atoms when making a molecule. Other than that, they’re completely different. Silicon is a metalloid; carbon is a nonmetal. Carbon is much lighter and more flexible, and it’s ten times more abundant in the universe.
If the idea of silicon based life is simply to replace carbon atoms with silicon, it wouldn’t work. Take breathing for example. We breath oxygen in, and exhale carbon dioxide. When a silicon based alien breaths in oxygen, it will have a hard time exhaling silicon dioxide; silicon dioxide is better known as quartz crystal.
I don’t remember any xenomorphs hacking up quartz crystals in the movies, but maybe they use silicon for something else. Carbon has to be part of their biochemistry anyway, or they wouldn’t be able to grow inside human hosts.
Humans are not only carbon based. We also depend on oxygen, nitrogen, hydrogen, phosphorus, and sulfur. Not only that, but we need traces of iron, sodium, potassium, etc as well. So maybe the xenomorphs can be carbon based and silicon based at the same time.