Valence Bond Theory: Water

In Valence Bond Theory (sometimes called the Localized Bonding Model), a σ bond betweeen two atoms is viewed as arising from direct overlap of an orbital from each atom. The orbitals used to describe the bond should be oriented directly at the other atom.

Consider the water molecule: H₂O. The central atom is oxygen. The Lewis structure for water involves two O-H σ bonds and two lone-pairs of electrons on the oxygen. In terms of the VSEPR Model, the oxygen has four electron groups, which means the valence electrons for the oxygen are sp³ hybrid orbitals.

Two of the oxygen sp³ oribtals accommodate the two lone pairs. The remaining two oxygen sp³ oribtals are used to describe the two O-H σ bond. Each O-H σ bond is constructed by overlap of an oxygen sp³ orbital with a hydrogen 1s orbital.

The isosurface viewer below illustrates Valence Bond Theory description of bonding in the water molecule. The small red sphere indicates the position of the oxygen nucleus. The two white spheres mark the positions of the two hydrogen nuclei.

First, examine each of the atomic orbitals. Note how two of the oxygen sp³ orbitals point directly at hydrogen atoms. The other two sp³ orbitals point away from the hydrogen atoms.

Next, overlay oxygen sp³ and hydrogen 1s orbitals to see the orbital overlap characteristic of a chemical bond. Compare the overlapping atomic orbitals with the σ orbitals for the water molecule.

Note that orbital positions are based upon the original view. The vertical axis is the z-axis. The molecule lies in the xz plane.

Questions

1. Why are the oxygen sp³ orbitals smaller than the hydrogen 1s orbitals?
2. In comparing the overlay of oxygen sp³ and hydrogen 1s orbitals with the corresponding O-H σ bond, do you see evidence of electron density being concentrated between the two nuclei in the O-H σ bond?