Molecular shape Lewis structure

Figure 10.7 Lewis structures and molecular shapes. Lewis structures do not indicate geometry. For example, it may seem as if two different Lewis structures can be written for CCIjPg, but a twist of the model (Cl, green F, yellow) shows that they represent the same moleoule.

Methane (CH4) and ethane (C2He) are the first two members of the alkane family. Figure 3.2 shows molecular formulas, Lewis structures, and ball-and-stick models for these molecules. The shape of methane is tetrahedral, and all H—C—H bond angles are 109.5°. Each carbon atom in ethane is also tetrahedral, and all bond angles are... 

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 ... 

Draw the Lewis structure of (a) NI and (b) BI, name the molecular shape, and indicate whether each can participate in dipole-dipole interactions. 

The Lewis stmcture of a molecule shows how the valence electrons are distributed among the atoms. This gives a useful qualitative picture, but a more thorough understanding of chemistry requires more detailed descriptions of molecular bonding and molecular shapes. In particular, the three-dimensional structure of a molecule, which plays an essential role in determining chemical reactivity, is not shown directly by a Lewis structure. 

Our approach to these molecules illustrates the general strategy for determining the electron group geometry and the molecular shape of each inner atom in a molecule. The process has four steps, beginning with the Lewis structure and ending with the molecular shape. 

Follow the four-step process described in the flowchart. Begin with the Lewis structure. Use this stracture to determine the steric number, which indicates the electron group geometry. Then take into account any lone pairs to deduce the molecular shape. 

Use the Lewis structure of CIF3 to determine the steric number of the chlorine atom. Obtain the molecular shape from the orbital geometry after placing lone pairs in appropriate positions. 

Elaving developed ideas about Lewis structures and shapes of molecules, we are now in a position to explore some of the important properties of covalent bonds. These properties provide revealing evidence about molecular shapes. 

As we describe in Section 94, the bond length of a covalent bond is the nuclear separation distance where the molecule is most stable. The H—H bond length In molecular hydrogen is 74 pm (picometers). At this distance, attractive interactions are maximized relative to repulsive interactions (see Figure 9-2). Having developed ideas about Lewis structures and molecular shapes, we can now examine bond lengths In more detail. 

Determine the Lewis structure and the molecular shapes of the carbon atoms of this molecule. Suggest a reason for the reactivity of C3 Hg. ... 

Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walters, P. (2002) The Shape and Structure of Proteins, Molecular Biology of the Cell, 4th edn. Garland Science, New York and London. 

We first give all the Lewis structures. From each, we deduce the electron-group geometry and the molecular shape. 

First we draw the Lewis structure of each species, then use these Lewis structures to predict the molecular shape. 

In PF6 there are 5 + (6 x 7) +1 = 48 valence electrons or 24 electron pairs. A plausible Lewis structure follows. Since there are six atoms and no lone pairs bonded to the central atom, the electron-group geometry and molecular shape are octahedral. 

First we must draw the Lewis structure for all of the species listed. Following this, we will deduce their electron-group geometries and molecular shapes following the VSEPR... 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

Draw Lewis structures for the following molecules and ions, and use VSEPR theory to predict the molecular shape. Indicate the examples in which the central atom has an expanded octet. 

Draw Lewis structures and indicate the molecular shape of the following. 

CD If possible for your material, draw a Lewis structure of the molecule or molecules on which your material is based. Predict the molecular shape using VSEPR theory. 

Although the discussions of the preceding molecules have been couched in valence bond terms (Lewis structures, hybridization, etc.), recall that the criterion for molecular shape (rule 2 above) was that the cr bonds of the central atom should be allowed to gel as far from each other as possible 2 at 180°. 3 at 120°, 4 at 109.5°, etc. This is (he heart of the VSEPR method of predicting molecular structures, and is, indeed, independent of valence bond hybridization schemes, although it is most readily applied in a VB context. 

STRATEGY Draw the Lewis structure and then decide how the bonding pairs and lone pairs are arranged around the central (nitrogen) atom (consult Fig. 3.4 if necessary). Identify the molecular shape from the layout of atoms and reference to Fig. 3.1. 

To determine the shape of a molecule, write the Lewis structure, then identify the arrangement of electron pairs and bonds in which the lone pairs are farthest from each other and from bonds name the molecular shape by considering only the locations of the atoms. Lone pairs distort the shape of a molecule to reduce lone pair-bonding pair repulsions. 

Self-Test 15.4B Draw the Lewis structure for XeOF4 and predict its molecular shape. 

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