Valence Shell Electron Pair Repulsion Model

Transcription

1 Valence Shell Electron Pair Repulsion Model Why? Molecules adopt a shape that minimizes their energy. In most cases simply considering the repulsive energy of electron pairs is sufficient to predict molecular shape. You can use this valence shell electron pair repulsion model to predict the molecular shape and to determine whether a molecule is polar or not. Scientists who work with molecules commonly use this model when they need to know the shape of a molecule. Learning Objectives λ Understand how molecular shape is predicted from the Lewis structure. λ Identify polar and nonpolar molecules. Success Criteria λ Ability to distinguish between the Lewis structure and the molecular shape. λ Accuracy in determining molecular shapes and identifying polar molecules. Resources Olmsted and Williams (Chemistry 3/e, Wiley, 2002) pp Prerequisites Lewis electronic structures, polar and nonpolar molecules New Concepts VSEPR model, polar and nonpolar molecules, molecular shape Definitions In your own words, write definitions of the above concept terms. Information The terms Lewis structure, electronic structure, electron arrangement, and electron geometry all are used to describe how the bonding and nonbonding electron pairs are positioned in a molecule. The terms molecular shape, molecular structure, and molecular geometry all are used to describe how the atoms are positioned relative to each other in a molecule.

2 Model: Methodology for Determining Molecular Geometries (or Structures) from the VSEPR Model Methodology Example For sulfur dioxide - Step 1: Draw the Lewis electronic structure. O S O Step 2: Count the number of bonds and nonbonding electron pairs around the central atom. Step 3: Molecules take a shape that minimizes their energy. Arrange the bonds and nonbonding electron pairs to maximize their separation, which minimizes the electron-electron repulsion energy. Step 4: Add the atoms in a way consistent with how the electrons are shared and space the nonbonding electron pairs as far apart as possible. First, minimize the number of interactions between nonbonding electron pairs at 90 o to each other, and then at 90 o to bonding pairs because such interactions increase the energy of the structure significantly. Step 5: Determine the molecular shape from the position of the atoms. Step 6: Identify whether the molecule is polar or not. 1 single + 1 double bond + 1 nonbonding pair = 3. This number is called the steric number. A steric number of 3 in step 2 means a trigonal planar electronic structure O S O The atoms are arranged in a nonlinear or bent shape. Oxygen is more electronegative than sulfur so SO2 is polar since the center of negative charge (halfway between the two oxygen atoms) is displaced from the center of positive charge (the sulfur atom).

3 Key Questions 1. How do you determine the Lewis electronic structure around the central atom in Step 1 in the preceding methodology? 2. Why are bonds and nonbonding electron pairs (aka lone pairs) spaced as far apart as possible in the structure? 3. According to Step 3 in the methodology, if you have two lone pairs and bonds to four atoms around a central atom, would you position the lone pairs at 90 o or 180 o to each other? Explain. 4. How would you describe the geometrical arrangement of the bonds and nonbonding electrons around sulfur in sulfur dioxide? 5. How would you describe the shape sulfur dioxide? (The terms linear and bent are commonly applied to triatomic molecules.) 5. Why are some triatomic molecules linear and some bent? 6. What three insights have your team gained about the shape of molecules by examining the model and responding to the key questions?

4 Exercises 1. Complete the illustrations in the following table to show the arrangement of bonds and electron lone pairs that minimizes the energy in each case. Your illustration represents the Lewis electronic structure of the molecule. The number of bonds and lone pairs is called the steric number. Number of Bonds and Lone Pairs Lewis Electronic Structure 2 linear Illustration of the Electronic Structure 3 trigonal planar 4 tetrahedral 5 trigonal bipyramid 6 octahedral

5 2. Use the VSEPR model to predict the shape of each of the following molecules and then sketch the molecule in the first column and the appropriate row of the table below. Label the molecule as polar or nonpolar. O 3 I3 - IF 6 + SbF5 COCl2 SeO 3 2- SiF 4 KrF 4 SF 4 ICl 3 BrF 5 Structure/Example linear Illustration bent trigonal planar

6 tetrahedral trigonal bipyramidal trigonal pyramidal

7 octahedral square planar square pyramidal

8 see-saw tee bent, linear, octahedral, trigonal bipyramidal, trigonal planar, trigonal pyramidal, tetrahedral, square planar, see-saw, tee, square pyramidal Problems 1. An article in a journal, Inorganic Chemistry, cites both BF 3 and PF 3 as examples of flat or planar molecules with bond angles of 120 o. Another article reports the FPF bond angle as 98 o. Which report is consistent with the VESPR model? Explain. 2. Is the shape of OCS like that of CO 2 or SO 2? Identify which are linear and which are bent.