Why does life have to be carbon-based?

Scientists have occasionally speculated that life could be based on an element other than carbon. Silicon, being the lightest element with an electronic structure analogous to that of carbon (having a half-filled outer shell with 4 unpaired electrons), is the most likely candidate mentioned. However, carbon's tendency to form the long chains and rings that form the basis for organic compounds that at some level of complexity begin to self-replicate is unique. Also, because older stars naturally produce carbon, along with nitrogen and oxygen (its neighbors on the periodic table), it is relatively abundant in the universe. Many astrophysicists who study the spectra of stars believe that complex chains and even rings of carbon appear in such unlikely places as stellar envelopes (e.g., in the form of PAHs, polycyclic aromatic hydrocarbons). When such compounds reach cooler regions of space where they can bond with readily available hydrogen, organic compounds as we know them are naturally formed.

Although other elements may form complex, covalently bonded structures, none has the rich molecular variety of carbon. It is the chemistry of carbon that allows us to consider the possibility of life "as we know it" in other parts of the Galaxy and the Universe beyond. We do not know whether Earth-like conditions exist elsewhere; but if they do, it is highly likely that life forms, if they exist, will be based on carbon.

One more point: The organic types of structures appearing in stellar envelopes are very hot and probably stripped of hydrogen, so that they are not themselves alive; it is only when carried off to a more hospitable environment, such as a much cooler planet 100 million miles away or so, that the kind of chemistry required by life becomes possible on a scale large enough to allow for stable development and replication. Water is also a factor, causing the hydrophobic proteins to clump together at all, and serving as a medium a conduit for new material, protection from temperature changes and harmful stellar radiation, etc.

It does not have to, but no one has seen anything that is not carbon based. Carbon has a few interesting properties that make it ideally suited as a basic for life. First, it is relatively abundant. Eleemtns have a tendency to become more rare as you go up in the periodic chart, and a high abundance means it is likely to be around when a few chemical reactions occured several billion years ago that lead to the emergence of life.

Carbon also has 4 valance electrons, which means it is capable of forming chains or lattice, an ideal property for building large molecules that will have bth the required stability and the capability to react; life requires a good balance of stability and reactivity.

If you look at the periodic table, you will notice that silicon is supposed to have similar characteistics to carbon, having 4 valance electrons as well. However, being heavier, that atom is likely to have slower reactions than carbon. It is also less attached to its electron than carbon is, so will more easily give them away and have ionic links rather than covalent bounds.

Boron can be though as another element that could have been the support for life, and boron compounds are plentiful; but it only has 3 oxidation levels. Nitrogen has a +5 oxidation number, but those are unstable -- that is why most explosives are nitrogen compounds. Explosives compounds are not that good a choice to form the basis of a life; and in the stable organic compounds, nitrogen will be found with its more stable -3 oxidation number.

This leaves carbon as the best choice.

This is due to the way in which carbon forms molecules and reacts with itself and other chemical compounds.

Only life as we know it is carbon based. This is probably because of the abundance of carbon on Earth. There is no reason to believe something that resembles life can exists with carbon.

Well, carbon has this amazing property to bond with the maximum number of elements and it can also combine with itself to form hundreds of millions of compounds each with a different chemical property.thus, all life is basically carbon based because it has the largest number of derivative compounds.

You been watching too many sci fi movies.

Better study and quit asking silly questions.