To account for the millions of molecules with tetrahedral shapes, the model proposes that the central atom undergoes sp3 hybridization. For example, in the carbon of methane, the filled 2s, two half-filled 2p's, and empty 2p mix and become four half-filled sp3 hybrid orbitals.
The model describes bonding in methane this way. In the molecule, the carbon's four sp3 hybrid orbitals point to the corners of a tetrahedron. The H orbitals overlap them to form four C-H bonds that are 109.5° apart.
For other shapes within a given electron-group arrangement, the model proposes lone pairs in one or more of the hybrid orbitals. For example, the trigonal pyramidal shape of ammonia arises when the 2s and the three 2p's of the central nitrogen mix and become four sp3 hybrid orbitals, one of which is filled with a lone pair.
Here we visualize the nitrogen atom undergoing sp3 hybridization. One of the tetrahedrally oriented sp3 hybrids is filled with a lone pair, and the H atoms overlap the other three to form three N-H bonds.
In the case of water, with its V-shape, the model proposes a situation similar to that for ammonia. The 2s orbital of the central O atom mixes with its three 2p's and become four sp3 hybrids, but now two of the hybrid orbitals are filled with lone pairs.
In the water molecule, the oxygen has undergone sp3 hybridization. Two of the sp3 orbitals are filled with lone pairs, and the H atoms overlap the other two to form two O-H bonds. |
