A saturated bond between carbon and hydrogen does not readily react with any chemical entity other than oxygen via combustion.
An unsaturated bond on the other hand is far more reactive to various other chemical entities.
Yes, carbon in any compound is an atom or an element. The nature of the bonds that if forms in any molecule dictates its behaviour. The bonds are important.
An atom is not the smallest unit of matter. All but the smallest atoms consist of protons and neutrons in their nucleus with electrons whizzing around them in well-ordered quantum orbitals. Protons and neutrons have significant mass and electrons just a tiny mass. Beyond this, particle physics gets rather complicated and very theoretical.
But at the end of they day, we all have to draw a line somewhere and say look, I just don't need/want to know any more!
F 5 ft 3. HW: 196 lbs. Primal SW (May 2011): 182 lbs (42% BF)... W June '12: 160 lbs (29% BF) (UK size 12, US size 8). GW: ~24% BF - have ditched the scales til I fit into a pair of UK size 10 bootcut jeans. Currently aligning towards 'The Perfect Health Diet' having swapped some fat for potatoes.