Molecular Geometry from Lewis Structures

Resonance is possible whenever a Lewis structure has a multiple bond and an adjacent atom with at least one lone pair. The arrows in the following generalized structures show how you can think of the electrons shifting as one resonance structure changes to another. 

To generate the second resonance structure from the first, we imagine one lone pair dropping down to form another bond, and pushing an adjacent bond off to form a lone pair. The arrows show this hypothetical shift of electrons, which leads to the resonance hybrid below. 

Draw all of the reasonable resonance struaures and the resonance hybrid for the carbonate ion, COp. A reasonable Lewis structure for the carbonate ion is 

You can find a more comprehensive description of resonance at the textbook s Web site. 

The shapes of molecules play a major role in determining their function. For example. 

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Section 12.4 Molecular Geometry from Lewis Structures... 

Plan To predict the molecular geometries, we draw their Lewis structures and count electron domains around the central atom to get the electron-domain geometry. We then obtain the molecular geometry from the arrangement of the domains that are due to bonds. 

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. 

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

As you look at this Lewis structure, notice that there are four pairs of electrons. There are three shared pairs, denoted by the lines, and one unshared pair, represented by the dots above the N atom. The unshared pair is also called a lone pair. Ammonia s four pairs of electrons are all valence electrons. The shape that allows these four pairs of electrons to be as far from each other as possible places them at the corners of a tetrahedron, as shown in Figure 9.6. This is called electron group geometry. The arrangement of the atoms is called molecular geometry, which in this case is pyramidal. 

A number of ions derived from the interhalogens are known. Write Lewis formulas and three-dimensional structures for the following ions. Name the electronic and molecular geometries of each, (a) IF4 (b) IC ... 

Tip-off In this chapter, you are given a Lewis structure for a molecule or polyatomic ion (or a formula from which you can draw a Lewis structure), and asked to (1) name the electron group geometry arormd one or more atoms in the structure, (2) draw a geometric sketch of the structrue, and/or (3) name the molecular geometry around one or more of the atoms in the structure. You will find in later chapters that there are other tip-offs for these tasks. 

That way the Lewis structure for carbon dioxide has now told us that in this molecule the carbon atom is placed in the centre. Each oxygen atom is double bonded to the carbon atom and two lone pairs are attached to each oxygen atom. But from the Lewis structure we know nothing about the actual molecular geometry. 

When we are to determine how many electron groups that surround an atom, the Lewis structure can be of great help (see the previous section 2.23 Lewis structure). From the Lewis structure of a given molecule you can simply count how many bonds and lone pairs that surround an atom. That way you have the number of electron groups. The VSEPR theoiy tells us that these electron groups will be placed as far apart as possible. In the following example we will use the VSEPR theory to predict the molecular geometries of a water molecule and a carbon dioxide molecule. That way we will discover why a carbon dioxide molecule is linear and why a water molecule is V-shaped. 

As we saw in Chapter 8, the localized electron model views a molecule as a collection of atoms bound together by sharing electrons between their atomic orbitals. The arrangement of valence electrons is represented by the Lewis structure (or structures, where resonance occurs), and the molecular geometry can be predicted from the VSEPR model. In this section we will describe the atomic orbitals used to share electrons and hence to form the bonds. 

Consider the molecule PF4CI. (a) Draw a Lewis structure for the molecule, and predict its electron-domain geometry, (b) Which would you expect to take up more space, a P — F bond or a P — Cl bond Explain, (c) Predict the molecular geometry of PF4CI. How did your answer for part (b) influence your answer here in part (c) (d) Would you expect the molecule to distort from its ideal electron-domain geometry If so, how would it distort ... 

A Each of these molecules has fluorine atoms attached to an atom from Group 1A or 3A to 6A. Draw the Lewis structure for each one and then describe the electron-pair geometry and the molecular geometry. Comment on similarities and differences in the series, (a) BF3 (b) CF4 ... 

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