3.3.1: The Hybrid Orbital Model
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In this model the orbitals involved in carbon–carbon bonding are considered to be hybrids or mixtures of atomic orbitals (see →). If carbon forms four bonds (and it does) then four bonding orbitals are needed. Carbon has available orbitals in the second (n = 2) quantum shell: the 2s, 2px, 2py, and 2pz. In an isolated carbon atom there are two full (2s and one 2p) and two half-filled (the other two 2p) atomic orbitals. When the carbon atoms form a bond these orbitals are somehow mysteriously transformed into four new bonding orbitals, which are called sp3 hybrid orbitals because they are a mixture of an s and three p orbitals. These sp3 orbitals exist only in the context of bonded carbon; they are not present in isolated carbon atoms. They spring into existence when one carbon atom interacts with another atom to form a bond; they are generated through the interaction. In the case of carbon the four electron clouds (bonds) move as far apart as possible to minimize the repulsions between them, adopting a tetrahedral configuration.