3.

Carbon -
Carbon is the 15th most abudant element in the Earth's crust, & the 4th most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all known life forms. In the human body carbon is the second most abundant element by mass  after oxygen.This abundance, together with the unique diversity of organic compounds and their unusual polymer-forming ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life, and diamonds are a polymorph of the element carbon.