Molecular shape polyatomic molecules

How does valence bond theory describe the electronic structure of a polyatomic molecule, and how does it account for molecular shape Let s look, for example, at a simple tetrahedral molecule such as methane, CH4. There are several problems to be dealt with. 

The VSEPR theory has its roots in the observation prior to 1940 that isoelectronic molecules or polyatomic ions usually adopt the same shape. Thus BF3, B03 C03, COF2 and NO3 are ail isoelectronic, and they all have planar triangular structures. As developed in more recent years, the VSEPR theory rationalises molecular shapes in terms of repulsions between electron pairs, bonding and nonbonding. It is assumed that the reader is familiar with the rudiments of the theory excellent expositions are to be found in most inorganic texts. 

You now know how to draw Lewis structures for molecules and polyatomic ions. You can use them to determine the number of bonding pairs between atoms and the number of lone pairs present. Next, you will learn to describe molecular structure and predict the angles in a molecule, both of which determine the three-dimensional molecular shape. 

In this section we consider clusters of some polyatomic molecules. The structure of such clusters is expected to depend greatly on both the shape of the molecules and the intermolecular forces. These characteristic properties will also govern the possibility that a molecular cluster will undergo structural transitions according to its size. It is worthwhile to note that diffraction patterns contain, in addition to structural information, various features related to dynamic effects such as translational and librational molecular motions, this making it more difficult to elucidate the cluster structure. On the other hand, size effects may be detected in both structural and dynamic properties. 

We will first discuss the basic ideas and application of these two theories. Then we will learn how an important molecular property, polarity, depends on molecular shape. Most of this chapter will then be devoted to studying how these ideas are applied to various types of polyatomic molecules and ions. 

Two theories go hand in hand in a discussion of covalent bonding. The valence shell electron pair repulsion (VSEPR) theory helps us to understand and predict the spatial arrangement of atoms in a polyatomic molecule or ion. It does not, however, explain hoav bonding occurs, ] ist where it occurs and where unshared pairs of valence shell electrons are directed. The valence bond (VB) theory describes how the bonding takes place, in terms of overlapping atomic orbitals. In this theory, the atomic orbitals discussed in Chapter 5 are often mixed, or hybridized, to form new orbitals with different spatial orientations. Used together, these two simple ideas enable us to understand the bonding, molecular shapes, and properties of a wide variety of polyatomic molecules and ions. 

Picosecond laser spectroscopy offers direct access to molecular vibrational (T,) and phase (Tj) relaxation as well as orientational dynamics of molecules, t,< . In contrast to experiments on vibrational relaxation in liquids, all those on the reorientational process have been confined to large polyatomic molecules, particularly dye molecules in probe solvents for pragmatic reasons. Since the slip boundary conditions are also a sensitive function of the shape of the molecules and solute-solvent interactions, there is some uncertainty in deciding whether or not it is the local interaction in terms of the solvation volume, or the boundary condition, or both, that varies for a given molecule in a range of liquids such as the... 

SECTION 9.3 The dipole moment of a polyatomic molecule depends on the vector sum of the dipole moments associated with the individual bonds, called the bond dipoles. Certain molecular shapes, such as linear AB2 and trigonal planar AB3, assure that the bond dipoles cancel, producing a nonpolar molecule, which is one whose dipole moment is zero. In other shapes, such as bent AB2 and trigonal pyramidal AB3, the bond dipoles do not cancel and the molecule will be polar (that is, it will have a nonzero dipole moment). 

Also, in Chapter 3 we introduced the concept of molecular-orbital theory to explain bonding in diatomic molecules. In Chapter 4, we will extend this useful quantum-mechanical concept to polyatomic molecules. In addition, in the final section of Chapter 4, we will examine how molecular shape and bonding affect the interactions of molecules with one another. 

The VSEPR model reliably predicts the geometry of many molecules and polyatomic ions. Chemists use the VSEPR approach because of its simplicity. Although there are some theoretical concerns about whether electron-pair repulsion actually determines molecular shapes, the assumption that it does leads to useful (and generally reliable) predictions. Example 4.1 illustrates the application of VSEPR. 

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