The Lewis electron-pair approach described in Chapter 8 can be used to predict the number and types of bonds between the atoms in a substance, and it indicates which atoms have lone pairs of electrons. This approach gives no information about the actual arrangement of atoms in space, however.
The specific three dimensional arrangement of atoms in molecules is referred to as molecular geometry. We also define molecular geometry as the positions of the atomic nuclei in a molecule. There are various instrumental techniques such as X-Ray crystallography and other experimental techniques which can be used to tell us where the atoms are located in a molecule. Using advanced techniques, very complicated structures for proteins, enzymes, DNA, and RNA have been determined. Molecular geometry is associated with the chemistry of vision, smell and odors, taste, drug reactions and enzyme controlled reactions to name a few.
Example 4.1.1: Carbon Tetrachloride
The Lewis structure of carbon tetrachloride provides information about connectivities, provides information about valence orbitals, and provides information about bond character.
However, the Lewis structure provides no information about the shape of the molecule, which is defined by the bond angles and the bond lengths. For carbon tetrachloride, each C-Cl bond length is 1.78Å and each Cl-C-Cl bond angle is 109.5°. Hence, carbon tetrachloride is tetrahedral in structure:
Molecular Geometries of \(AB_n\) molecules
Molecular geometry is associated with the specific orientation of bonding atoms. A careful analysis of electron distributions in orbitals will usually result in correct molecular geometry determinations. In addition, the simple writing of Lewis diagrams can also provide important clues for the determination of molecular geometry. Molecular shapes, or geometries, are critical to molecular recognition and function. Below are examples of geometries with a central atom \(A\) is bonded to two or more \(X\) atoms. As indicated in several of the geometries below, non-bonding electrons \(E\) can strongly influence the molecular geometry of the molecule; this is discussed in more details in Section 9.2.
|1 lone pair of electrons|
|2 lone pairs of electrons|
These structures can generally be predicted, when A is a nonmetal, using the "valence-shell electron-pair repulsion model (VSEPR) discussed in the next section.