Chapter 10 Lecture

Chapter 10 Lecture - Chapter 10 Lecture 04/04/02 Dr SJ...

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Chapter 10 Lecture 04/04/02 Dr SJ Brois Molecular Structure and Bonding Theory Chemists have devised models that allow us to predict molecular structures fairly accurately. [I] A simple model based on Lewis structures is a prime example. [II] Two other theories explain bonding in greater detail: Valence bond,VB, theory, and molecular orbital, MO, theory VB and MO describe molecules by combining the atomic orbitals of each of the atoms. [I] First, the chemist takes a Lewis structure and predicts its shape by resorting to the VSEPR MODEL: VALENCE SHELL ELECTRON PAIR REPULSION MODEL Its basic premise: Electron pairs in atoms tend to repel each other VSEPR Rule #1 A molecule assumes a shape that minimizes repulsions between valence shell electrons by staying as far apart as possible in the molecule 1
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.Let’s start with a simple Lewis structure to better define Rule #1 and introduce the idea of a steric number VSEPR predicts the shape of the central atom which is bonded to at least two other atoms Starting with a Lewis structure, we count the # of lone pairs on the central atom and the # of atoms bonded to it. This sum is called the steric number which is defined as follows: Steric number = # of lone pairs on central atom (Be) + # of atoms bonded to central atom The steric # determines the electron pair arrangement ( epa) which is the shape that maximizes the distance between ep’s. see figure 10.1 Its easiest to show how VSEPR works with only 2 electron pairs around the central atom as in :Cl-Be-Cl: The central atom Be, has 2 bonded atoms and zero electrons pairs, so it has a steric # of 2 which requires a linear geometry. Thus the model predicts a linear shape with a Cl-Be-Cl bond angle of 180 degrees which affords the maximum separation of EP’s. 2
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determining the steric # of the central atom. The VSEPR model only predicts the shape about the central atom. For central atoms bonded to three atoms, the steric # is 3, so, the EP arrangement is trigonal planar as in boron trifluoride: :F---B---F: :F: For central atoms where the total of bonded atoms plus lone pairs is 4 the steric # is 4 and the EPR is minimized by a tetrahedral shape wherein the angles are 109.5 ° as shown in methane whose H’s are all equivalent: H H--C--H H The EP arrangement for a central atom bonded to 5 atoms and lone pairs requires a steric # of 5 and is called a trigonal bipyramid as in PF 5 In contrast to other EP arrangements, the F’s exist in two different environments, one being equatorial, the other axial. Finally, the EP shape for a central atom with a total of 6 bonded atoms and lone pairs,a steric # of 6, is octahedral ,as in SF 6 wherein F’s are equivalent. Central Atoms with Lone Pairs
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Chapter 10 Lecture - Chapter 10 Lecture 04/04/02 Dr SJ...

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