Why, out of four orbitals of carbon, two orbital remained unhybrid in sp3?

BRY2K

The site below might help, here is an excerpt that I *think* responds to your question: "If lone pair electrons are present on the central atom, these can occupy one or more of the sp3 orbitals. This causes the molecular geometry to be different from the coordination geometry, which remains tetrahedral. In the ammonia molecule, for example, the nitrogen atom normally has three unpaired p electrons, but by mixing the 2s and 3p orbitals, we can create four sp3-hybrid orbitals just as in carbon. Three of these can form shared-electron bonds with hydrogen, resulting in ammonia, NH3. The fourth of the sp3 hybrid orbitals contains the two remaining outer-shell electrons of nitrogen which form a non-bonding lone pair. In acidic solutions these can coordinate with a hydrogen ion, forming the ammonium ion NH4+. Although no bonds are formed by the lone pair in NH3, these electrons do give rise to a charge cloud that takes up space just like any other orbital. " See here: http://www.chem1.com/acad/webtext/chembond/cb06.html Nov 13 09, 11:36 AM

looney_tunes

sp3 hybridization does NOT have 2 unhybrid orbitals - sp3 hybridization involves 4 hybrid orbitals; sp2 involves 3 hybrid orbitals and 1 unhybrid orbital; sp involves 2 hybrid orbitals and 2 unhybrid orbitals. http://en.wikipedia.org/wiki/Orbital_hybridisation Nov 13 09, 2:31 PM