hybridization
Quantum-mechanical calculations suggest why the observed bond angles in H2O differ from those predicted by the overlap of the 1s orbital of the hydrogen atoms with the 2p orbitals of the oxygen atom. The mathematical expression known as the wave function, , contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals, LCAO, a technique that we will encounter again later . The new orbitals that result are called hybrid orbitals. The valence orbitals in an isolated oxygen atom are a 2s orbital and three 2p orbitals. The valence orbitals in an oxygen atom in a water molecule differ; they consist of four equivalent hybrid orbitals that point approximately toward the corners of a tetrahedron Figure 8.7 . Consequently, the overlap of the O and H orbitals should result in a tetrahedral bond angle 109.5 . The observed angle of 104.5 is experimental evidence for which quantummechanical calculations give a useful explanation: Valence bond theory must include a hybridization component to give accurate predictions. Note that orbitals may sometimes be drawn in an elongated balloon shape rather than in a more realistic plump shape in order to make the geometry easier to visualize.