I'm continuing with the chemistry series for now.
Carbon is unique among the elements because of its linking ability. No other element has the same ability to form long chains, multiple branches, and linkable rings. Because of this linking ability, it can be found in both compounds and in solid form.
It is fourth on the list of the most abundant elements in the solar system, largely because of the large quantities of methane in the atmospheres of Jupiter, Saturn, Uranus, and Neptune, as well as its presence in the Sun. It's thought to be found in a form like soot in comets and some asteroids.
It's not one of the most abundant elements on earth, very possibly because the lighter carbon-containing compounds were driven away before the earth had formed or while it was forming, but it is quite abundant. Most of earth's supply is found in compounds contained in rock, with a small fraction concentrated as coal. A small fraction of the earth's carbon is found (in combined form) in the atmosphere. However, this small amount is vital to the existence of life, because it is trapped and incorporated into plant life, and then animal life, and re-released into the atmosphere in processes of respiration.