Just off the top of my head, I reckon that:
The valency of Carbon is four (Tetravalent). The Lewis Dot Structure of Carbon consists of four unpaired electrons in the valence (outermost) region of the atom. These four bonding pairs could take the form of single, double, or triple co-valent bonds as long as the total adds up to four co-valent bonds. If one looks at the electronic configuration of a Carbon atom,1s2, 2s2, 2p2 one observes that there are only two unpaired electrons that could presumably be able to form a total of two bonding pairs when bonded to other atoms, but we have just said that most Organic compounds have Carbon in the Tetravalent state. Why is Carbon Tetravalent usually instead of Divalent as we might expect looking at the electronic configuration? Linus Pauling in the 1950's attempted to answer that question by proposing the Theory of Hybridization of Atomic Orbitals. He explained that Carbon atoms undergo one of three types of hybridization depending upon the energy available. If Carbon atoms underwent sp3 hybridization then it could form four single bonds with other atoms. If the Carbon atom underwent sp2 hybridization then it would form one double and two single bonds with other atoms. If a Carbon atom underwent sp hybridization then it would form a triple and a single bond with other atoms. In each case the carbon atom would have four unpaired electrons in the hybridized state explaining why Carbon is usually Tetravalent. Silicon is also said to be sp3 hybridized.
Sorry to be so brief lads
