Molecular shape linear arrangement

Can Lewis structures provide us with information regarding the actual spatial arrangements of atoms within molecules To explore this question, let us invent a triatomic molecule and call it XY2, in which X is the central atom with bonds to each Y atom. Each bond is polar since the joined atoms have different electronegativities. Therefore each bond contributes to the overall dipole moment of the molecule. Consider two possibilities for the molecular shape, linear and bent ... 

Having introduced methane and the tetrahedron, we now begin a systematic coverage of the VSEPR model and molecular shapes. The valence shell electron pair repulsion model assumes that electron-electron repulsion determines the arrangement of valence electrons around each inner atom. This is accomplished by positioning electron pairs as far apart as possible. Figure 9-12 shows the optimal arrangements for two electron pairs (linear),... 

The secondary structure describes the molecular shape or conformation of the polymer chain. For most linear polymers this shape approaches a helical or pleated skirt (or sheet) arrangement depending on the nature of the polymer, treatment, and function. Examples of secondary structures appear in Figure 2.13. 

Figure 10.4 The single molecular shape of the linear electron-group arrangement. The key (bottom) for A, X, and E also refers to Figures 10.5, 10.6, 10.8, and 10.9.

The Molecular Shape with Two Electron Groups (Linear Arrangement)... 

Fig. la shows spherical particles adhering to each other in more or less linear arrangement, Fig. lb rodlet shaped particles building up a similar continuous framework. In Fig. Ic the case of linear macro-molecules forming a framework consisting of molecular chains with junction points of a crystalline nature is represented. Fig. Id shows a typical case of gel formation by chemical cross-linking of dissolved linear macromolecules. Such a case can, e. g., be reali2 ed if a rubber solution is vulcanised... 

Water is known to have the geometric shape known as bent or V-shaped. Carbon dioxide exhibits a linear shape. BF3 forms a third molecular shape called trigonal planar since all the atoms lie in one plane in a triangular arrangement. One of the more common molecular shapes is the tetrahedron, illustrated by the molecule methane (CH4). 

Figure 10.3 The single molecular shape of the linear electron-group arrangement.

Three of the five basic molecular shapes are linear, trigonal planar and tetrahedral. Table 4.9 shows the arrangement of the electron pairs (charge centres) that results in minimum repulsion and the basic shapes of the molecules. The two other basic shapes adopted by molecules, trigonal bipyramidal and octahedral, are discussed in Chapter 14. 

In a typical problem, we will be asked to consider a number of geometrical arrangements of the cluster A, where A is an atom or a fragment, predict the one which constitutes the global minimmn, and explain why this particular geometry is favored over the others. The molecular shapes which will be at the focus of our attention will be linear or chain, two-dimensional cyclic, and three-dimensional polyhedral geometries. Hence, restricting our attention fo an sp AO basis, we classify our AO s.in such a way so as to differentiate between Surface AO s and Needle AO s as follows ... 

The VSEPR model is used to determine molecular shape. It assumes arrangements that minimize repulsion of electron pairs around the central atom. The presence of four bonding pairs produces a tetrahedral arrangement. Three such pairs produce a trigonal planar shape, unless there also is a lone pair on the central atom, in which case the shape is trigonal pyramidal. Two bonding pairs produce a linear shape, but if there also are two lone pairs on the central atom, the shape will be bent. 

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