The smallest unit of Carbon is one atom. Each benzene ring as pictured represents one atom. Molecules are compounds of two or more element's atoms. I don't know how a single C atom is cleaved from its char brethren undergound, but I would assume there are enzymes or such, which promote, cause, or catalyze the necessary reaction and/or bonding with other elements. Whatever else is true, it happens: char is decayed over time into subsequent compounds, which compounds re-enter the food chain of plants when that occurs underground. Above ground, prolonged exposure to air, water, and sunlight, and the expansion and contraction of temperature changes promote disintegration, atom by atom. As an aside, it is improbable that O alone is capable of bonding alone and by its own forces because of the nearly constant exposure to O that char has above and below ground, such that a single event would make CO, which is a gas and would escape into the atmosphere before further bonding could occur.