Valence Shell Electron Pair Repulsion Model

Transcription

1 Activity 22 Valence hell Electron Pair Repulsion Model Why? Molecules adopt a shape that minimizes their energy. In many cases it is possible to predict the geometry of a molecule simply by considering the repulsive energy of electron pairs. You can use this valence shell electron pair repulsion model (VEPR) to predict shapes and determine whether or not a molecule is polar. cientists commonly use this model when they need to predict or estimate the shape of a molecule. Learning bjective Understand how molecular shape is predicted from the Lewis structure uccess Criterion Accuracy in predicting molecular shapes Prerequisite Activity 21: Lewis Model of Electronic tructure Information The terms Lewis structure, electronic structure, electron arrangement, and electron geometry 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 are used to describe how the atoms are positioned relative to each other in a molecule. Activity 22 Valence hell Electron Pair Repulsion Model 145

2 Model: Methodology for Determining Molecular Geometries (hapes or tructures) from the VEPR Model Table 22.1 Methodology Example or sulfur dioxide: tep 1: Draw the Lewis electronic structure. tep 2: Count the number of bonds and nonbonding electron pairs around the central atom. tep 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. 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 minimizes the energy: tep 4: Add the atoms in a way that is consistent with how the electrons are shared, and put the nonbonding electron pairs as far apart as possible. tep 5: Determine the molecular shape from the position of the atoms. The atoms are arranged in a nonlinear or bent shape. 146 oundations of Chemistry

3 Key Questions 1. In tep 1 in the preceding methodology, how do you determine the Lewis electronic structure? 2. Why are bonds and nonbonding electron pairs (aka: lone pairs) spaced as far apart as possible in the structure? 3. According to tep 4 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 90º or 180º to each other? Explain. 4. How would you describe the geometrical arrangement of the bonds and lone pairs around sulfur in sulfur dioxide? 5. How would you describe the shape of sulfur dioxide? (The terms linear and bent are commonly applied to triatomic molecules like 2.) 6. ome triatomic molecules are linear. What feature of 2 leads to the bent geometry? 7. What three insights have your team gained about the shape of molecules by examining the model and responding to the key questions? Activity 22 Valence hell Electron Pair Repulsion Model 147

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 tructure Illustration of the Electronic tructure Table linear 3 trigonal planar 4 tetrahedral 5 trigonal bipyramid 6 octahedral 148 oundations of Chemistry

5 2. Use the VEPR model to predict the shape of each of the following molecules then sketch the molecule in the first column and the appropriate row of the table below. 3 I3 I6+ b5 C2 e32 i4 Kr4 4 I3 Br5 Table 22.3 tructure/example Illustration linear I- I - I bent trigonal planar tetrahedral i Activity 22 Valence hell Electron Pair Repulsion Model 149

6 tructure/example Illustration trigonal bipyramidal b trigonal pyramidal 2- e - octahedral + I+ 150 oundations of Chemistry

7 tructure/example Illustration square planar Kr square pyramidal Br see-saw tee I Activity 22 Valence hell Electron Pair Repulsion Model 151

8 Problems 1. An article in a journal, Inorganic Chemistry, cites both B 3 and P 3 as examples of flat or planar molecules with bond angles of 120. Another article reports the P bond angle as 98. Which report is consistent with the VEPR model? Explain. B 120 P <109.5 B 3 is an example of a trigonal planar molecule, as shown above. The B angles are predicted to be 120. The P 3 molecule is predicted to be trigonal pyramidal in shape, which is not planar, as stated in the first article. However, the P bond angles of 98 are consistent with the trigonal pyramidal shape. The electronic structure of P 3 is predicted to be tetrahedral, and one might predict the P bond angles to be However, the presence of the lone pair on the phosphorus will decrease the P bond angle, and 98 is consistent with this interpretation. 2. Is the shape of C like that of C 2 or 2? Identify which are linear and which are bent. C C is bent whereas C and C 2 are linear as shown above. 152 oundations of Chemistry