Video Credit- PBS Space Time, YouTube
In many works of science fiction, aliens are described as being silicon-based life forms. Life on Earth is carbon-based, and evidence points to the fact that most life in the Universe will be carbon-based for several reasons.
Silicon is a very common element found on Earth in rocks in the soil. Like carbon, silicon can form four bonds and thus can create rings and a variety of scaffold molecules. As a student of biology, organic chemistry highlights the way that carbon's four bonds can help create molecules in such a way that stability can be preserved. Carbon can form strong bonds that enable life to thrive and to be resilient. Molecules such as benzene rings and alcohol serve a purpose that could not easily be replicated by Silicon for several reasons.
Several pieces of evidence point away from silicon-based life forms. Silicon does not interact well with water, and silicon based life would respire sand (SiO2) instead of the gas CO2 as carbon-based life forms do. Oxygen forms strong bonds with silicon, and thus a large amount of sand would accumulate inside of silicon-based lifeform's cells. About 2.2 pounds of C02 are respired by an adult human daily, which would be disastrous for a Silicon-based lifeform who might respire a similar volume of silicon-dioxide (solid sand).
Silicon-based life is improbable, but ammonia-based life or even Sulfur-based life is a possibility. Carbon is a great element to create rings and scaffold structures that facilitate the evolution of lifeforms. I enjoy the science fiction stories with Silicon-based lifeform's, but imagine their existence to be improbable.