There are now more than ten million organic compounds known by chemists. Many more undoubtedly exist in nature, and organic chemists are continually creating (synthesizing) new ones. Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms, and because the carbon atom is just the right, small size to fit in comfortably as parts of very large molecules.Having the atomic number 6, every carbon atom has a total of six electrons. Two are in a completed inner orbit, while the other four are valence electrons—outer electrons that are available for forming bonds with other atoms.The carbon atom's four valence electrons can be shared by other atoms that have electrons to share, thus forming covalent (shared-electron) bonds. They can even be shared by other carbon atoms, which in turn can share electrons with other carbon atoms and so on, forming long strings of carbon atoms, bonded to each other like links in a chain. Silicon (Si), another element in group 14 of the periodic table, also has four valence electrons and can make large molecules called silicones, but its atoms are too large to fit together into as great a variety of molecules as carbon atoms can.