The Shape of Simple Molecules

The crucial assumptions that may lead to the derivation of possible shapes for molecules such as CH, (CH)2, CH2 and (CH2)2 on the basis of orbital angular momentum arguments, are as follows  

On the basis of the simple rules defined above, the shapes of all molecules can in principle be predicted by logical procedures. It will be argued that any quantitative scheme that takes into account, not only the energy levels of electrons in molecules, but also their angular momenta must yield a comparable result that contains a framework for the definition of three-dimensional molecular shape. A few hydrides and other simple molecules will be discussed to demonstrate the principle. 

The dihydrogen molecule (s1 + s1) is formed through the interaction of s-electrons only. There is no angular momentum to impart structure to the diatomic molecule, which hence cannot be described in classical geometrical terms other than a spherically symmetrical distribution of electron and proton densities. It has no shape, no bond and, unless it interacts with external fields, no geometrical features. Compounds such as LiH, Li2, etc. belong to the same class of amorphous molecules. 

Boron is the simplest atom that can form structured molecules due to a p-electron in the valence shell (s2 1). This arrangement allows a directed bond H-B along 2  

For BH3 the single p-electron defines the special z direction. For paired spins this direction must coincide with a B-H bond. The three H atoms can therefore not be equivalent and the most likely H(s)(sp)B(sp)(s)H linear array has an unpaired s electron on B that interacts with the third H atom which is smeared out along an equatorial annulus that defines the overlap of the circulating H(ls) density with the B(2s) shell. 

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How can the shapes of simple molecules be explained in terms of electi on pair repulsions Your answer should include at least one example from each of four different shapes. 

FIGURE 3.1 The names of the shapes of simple molecules and their bond angles. Lone pairs of electrons are not shown because they are not included when identifying molecular shapes. 

The Lewis structures encountered in Chapter 2 are two-dimensional representations of the links between atoms—their connectivity—and except in the simplest cases do not depict the arrangement of atoms in space. The valence-shell electron-pair repulsion model (VSEPR model) extends Lewis s theory of bonding to account for molecular shapes by adding rules that account for bond angles. The model starts from the idea that because electrons repel one another, the shapes of simple molecules correspond to arrangements in which pairs of bonding electrons lie as far apart as possible. Specifically ... 

Figure 3.7 summarizes the shapes of simple molecules that result in them being polar or nonpolar. 

How do the properties of liquids and solids compare, and how can you use bonding theory to predict the shape of simple molecules ... 

From the point of view of the chemist, dipole moments have been most important in indicating the shapes of simple molecules. The molecules CO2, CS2, HgCl2, and HgBr2 have zero dipole moments, whereas the molecules H20, S02, and (CgHs Te have dipole moments between 1.0 and 2.0 D units, indicating that the latter three molecules are bent, whereas the first four are linear. Sometimes the presence of a dipole is represented on the structural formula of a molecule by an arrow, pointing from the more positive end of the molecule to the more negative end. Thus ... 

VSEPRT seems to work for simple structures but surely there must be more to it than this Indeed there is. If we really want to understand why molecules adopt the shapes they do, we must look at the atoms that make up the molecules and how they combine. By the end of this chapter, you should be able to predict or at least understand the shapes of simple molecules. For example, why are the bond angles in ammonia 107°, while in hydrides of the other elements in the same group as nitrogen, PH3, AsH3, and SbH3> they are all aronnd 90° Simple VSEPRT would suggest tetrahedral arrangements for each. 

Valence shell electron pair repulsion theory (VSEPR) provides a method for predicting the shape of molecules, based on the electron pair electrostatic repulsion. It was described by Sidgwick and Powell" in 1940 and further developed by Gillespie and Nyholm in 1957. In spite of this method s very simple approach, based on Lewis electron-dot structures, the VSEPR method predicts shapes that compare favorably with those determined experimentally. However, this approach at best provides approximate shapes for molecules, not a complete picture of bonding. The most common method of determining the actual stmctures is X-ray diffraction, although electron diffraction, neutron diffraction, and many types of spectroscopy are also used. In Chapter 5, we will provide some of the molecular orbital arguments for the shapes of simple molecules. 

The rules and principles of molecular geometry accurately predict the shapes of simple molecules such as methane (CH4), water (H2O), or ammonia (NH3). As molecules become increasingly complex, however, it becomes very difficult, but not impossible, to predict and describe complex geometric arrangements of atoms. The number of bonds between atoms, the types of bonds, and the presence of lone electron pairs on the central atom in the molecule critically influence the arrangement of atoms in a molecule. In addition, use of valance shell electron pair repulsion theory (VSEPR) allows chemists to predict the shape of a molecule. 

The shapes of simple molecules and ions of non-transition elements... 

There are basically three ways in which the shapes of simple molecules of the main group elements have been explained over the years. 

These chapters introduce you to the two main types of bonding found in nature ionic bonding and cov ent bonding. 1 show you how to predict the formulas of ionic compounds (salts) and how to name them. 1 explain covalent bonding, how to draw Lewis structural formulas, and how to predict the shapes of simple molecules. 1 tell you about chemical reactions and show you the various general types. In addition, 1 cover chemical equilibrium, kinetics, and electrochemistry — batteries, cells, and electroplating. 

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