What qualifies an element to serve as a basis for life?

I'm not sure I understand the question. Could you be more specific?

Like in the case of silicon. Why do scientists think it could serve as an alternative molecule to carbon for life?

Because Silicon is in the same group as Carbon. They have 4 electrons in their outer energy level, and they "need" 4 electrons. So they can form easily 4 links with other atoms, which make them very good candidates to be the basis of large, complex molecules.

 

So, what qualifies an element ? It's flexibility, it's capacity to share electrons to form complex structures. Also, light elements are generally more common, so it's probably a plus.

Ah, I see. Okay. What other elements have been considered as a possible basis for life? Could germanium be, since it's in the same group as carbon and silicon?

Noble gases have a very stable configuration where their outside electron shell is filled, specifically forming an octet. Most elements on the periodic table don't completely fill their outside shell, but they seek the stability of the noble gas configuration. So they bond with other atoms to fill their shell and make an octet. This bond between atoms enables two atoms to share electrons and fill their outside shells. They thus acquire the octet (which is simply 8 electrons in their outside orbital) they seek.

 

Carbon has 4 electrons and seeks to have 8 like a noble gas. This means that it is able to bind to 4 different atoms and share 4 different electrons in order to obtain the octet arrangement. This is important to life because since carbon is able to bind to so many different atoms that many different kinds of complex molecules can form based on carbon. If you look at the periodic chart you will see that silicon is below carbon. This means that it has the same number of electrons in its outside shell and it is able to bind 4 atoms at once as well.

I was looking into this a while ago, this article was an interesting read.

 

http://en.wikipedia.org/wiki/Non-carbon_biology

Thanks!