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The valence orbitals in an isolated oxygen atom are a 2 s orbital and three 2 p orbitals. The new orbitals that result are called hybrid orbitals.

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). When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. Quantum-mechanical calculations suggest why the observed bond angles in H 2O differ from those predicted by the overlap of the 1 s orbital of the hydrogen atoms with the 2 p orbitals of the oxygen atom.

This is not consistent with experimental evidence. Figure 1. The hypothetical overlap of two of the 2 p orbitals on an oxygen atom (red) with the 1 s orbitals of two hydrogen atoms (blue) would produce a bond angle of 90°. The prediction of the valence bond theory model does not match the real-world observations of a water molecule a different model is needed. Experimental evidence shows that the bond angle is 104.5°, not 90°. If this were the case, the bond angle would be 90°, as shown in Figure 1, because p orbitals are perpendicular to each other. Valence bond theory would predict that the two O–H bonds form from the overlap of these two 2 p orbitals with the 1 s orbitals of the hydrogen atoms. Oxygen has the electron configuration 1 s 22 s 22 p 4, with two unpaired electrons (one in each of the two 2 p orbitals). As an example, let us consider the water molecule, in which we have one oxygen atom bonding to two hydrogen atoms. However, to understand how molecules with more than two atoms form stable bonds, we require a more detailed model. Thinking in terms of overlapping atomic orbitals is one way for us to explain how chemical bonds form in diatomic molecules.