Section 8.13 Molecular Hybridization Structure: The VSEPR Model

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Curtis McCarthy
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1 Molecular Hybridization Structure: The VSEPR Model Covalent bonds are formed by the sharing of electrons; orbitals overlap to allow for this sharing. The mixing of two or more atomic orbitals of an atom forms hybrid orbitals. This process is called hybridization. Predicting hybridization is easy just count the total number of steric numbers (domains). The AP test does not cover hybridization past 4 domains.

2 The total number of steric numbers (also known as substituents - bonding plus non-bonding groups) is equal to the number of atomic orbitals that participate in the hybrid orbital. # of substituents (steric numbers) Hybridization Example Molecule 2 sp CO 2 3 sp 2 CH 3 4 sp 3 CH 4, NH 3, H 2 O 5 sp 3 d PCl 5, I 3-6 sp 3 d 2 SF 6

3 The sp hybrid orbital In sp hybridization, only one p orbital is mixed with the s orbital Example: BeF 2 Steric Number: 2 Electron configuration of Be: 1s 2 2s 2 Electron configuration of F:1s 2 2s 2 2p 5

4 In the ground state, Be has no unpaired electrons so how can the Be atom form a covalent bond with a fluorine? Be obtains an unpaired electron by moving one electron from the 2s orbital to the 2p orbital resulting in two unpaired electrons, one in a 2s orbital and another in a 2p orbital Be F

The Be atom can now form two covalent bonds with fluorine atoms Be F Although we would not expect these bonds to be identical (one is in a 2s electron orbital, the

5 The Be atom can now form two covalent bonds with fluorine atoms Be F Although we would not expect these bonds to be identical (one is in a 2s electron orbital, the other is in a 2p electron orbital), the structure of BeF 2 is linear and the bond lengths are identical The 2s and 2p electrons produced a "hybrid" orbital for both electrons

6 Molecular The sp 2 hybrid Structure: orbital The VSEPR Model In sp 2 hybridization, two p orbitals are mixed with the s orbital to generate three new hybrids Example: BF 3

7 Molecular The sp 3 hybrid Structure: orbital The VSEPR Model In sp 3 hybridization, all three p orbitals are mixed with the s orbital to generate four new hybrids Example: CH 4

9 Molecular Sigma (σ) Structure: and Pi Bonds The VSEPR (π) Model Sigma bond: The first bond made with any other atom Made from hybridized orbitals s-s, s-p, or p-p head-on overlap between nucleus Allows for free rotation Pi bond: Any 2 nd or 3 rd bond made with any other atom Made from leftover p orbitals parallel, sideways p-p overlap, nucleus above or below overlap Weaker bond than sigma Fixed rotation Single bond 1 sigma bond Double bond 1 sigma bond and 1 pi bond Triple bond 1 sigma bond and 2 pi bonds

10 Molecular Sigma (σ) Structure: and Pi Bonds The VSEPR (π) Model How many σ and π bonds are in the acetic acid (vinegar) molecule CH 3 COOH? H O H C C O H σ bonds = = 7 π bonds = 1 H

11 Given the structural formula for propyne: 1. Indicate the hybridization of each carbon atom in the structure above. 2. Indicate the total number of sigma (σ) and pi (π) bonds in the molecule.

12 Bond length and strength The more electrons that are involved in bonding, the shorter the bond length and the stronger the bond (meaning higher bond energy). Problem: Is the bond length between the two carbon atoms shorter in C 2 H 6, C 2 H 4, or C 2 H 2. Why? Problem: Use the bonding model to account for the fact that all the bond lengths in SO 3 are identical and are shorter than a sulfur-oxygen single bond.

13 Problem A. Draw the Lewis electron-dot structures for CO 2-3, CO 2 and CO. Which of the three species has the shortest C-O bond length? Explain the reason for your answer. Account for the fact that the carbon-oxygen bond length in CO 3 2 is greater than the carbon-oxygen bond length in CO 2.

CHAPTER TEN MOLECULAR GEOMETRY MOLECULAR GEOMETRY V S E P R CHEMICAL BONDING II: MOLECULAR GEOMETRY AND HYBRIDIZATION OF ATOMIC ORBITALS

CHAPTER TEN MOLECULAR GEOMETRY MOLECULAR GEOMETRY V S E P R CHEMICAL BONDING II: MOLECULAR GEOMETRY AND HYBRIDIZATION OF ATOMIC ORBITALS CHAPTER TEN CHEMICAL BONDING II: AND HYBRIDIZATION O ATOMIC ORBITALS V S E P R VSEPR Theory In VSEPR theory, multiple bonds behave like a single electron pair Valence shell electron pair repulsion (VSEPR)