Valence & Hybridization (I)
The number of atoms which are typically bonded to a given atom is termed the valence of that atom. Hydrogen would have a valence of one, oxygen would have a valence of two, nitrogen and boron would have a valence of three, and carbon would have a valence of four. Chemical analysis of simple hydrocarbons clearly demonstrates that carbon has a valence of four; that is, carbon forms four bonds and the simplest hydrocarbon (methane) has a molecular formula of CH4. The arrangement of these atoms in space, however, is not immediately apparent, and at least three possibilities should be considered as: square planar, in which the four hydrogens occupy the corners of a square, centered about the carbon, pyramidal, in which the carbon and three hydrogens occupy a plane with carbon in the middle and hydrogens at the vertices of a triangle, and tetrahedral, in which the carbon is in the center of a regular tetrahedron and hydrogens are at each vertex.
The orbital description of carbon, being 1s2 2s2 2p2, would suggest that, in order to form four bonds, a 2s electron must be promoted to the empty p-orbital to provide an electronic structure as shown below. Recalling the geometry of the three p-orbitals, the pyramidal geometry might seem to be favored, since the three p-orbitals would form bonds at 90 angles, with the s-orbital on the opposite face. This can easily be shown to be incorrect, however, by simply examining the number of molecules having the molecular formula CH2Cl2. As shown below, if carbon was square-planar, there should be two forms of the molecule CH2Cl2 (molecules having the same molecular formula, but different structures are termed isomers), one in which the two chlorines are on the same face of the square, and one in which they are in opposite corners. If carbon was pyramidal, again two isomers would be predicted, one in which both chlorines are in the trigonal plane. (continued).
