General Organic Chemistry
Carbon is an necessary element of organic substance; it has four electrons in its outer most shell.
As per to the ground state electronic arrangement of carbon, carbon is divalent. Tetravalency of carbon may be described by promoting one of the 2s2- electrons to the unoccupied 2pz1 atomic orbital.
The four valencies of carbon molecule are similar and they are symmetrically arranged around the carbon molecule. Illustrated by Le Bell and Van't Hoff the four valencies of carbon do not lie in one surface. They are going in the direction of the corners of a regular tetrahedron along with carbon atom at the centre and the angle among any two valencies is 109o28.