Review: Which particle contains the S – O bond with the higher dissociation energy? Sulfur trioxide, SO3 or the sulfite ion, SO3-2

Objective: Valence bond theory is 1 of 2 main theories of how molecules are held together. Both are accurate descriptions, but for different purposes.

Valence Bond Theory: Purpose: Very good at developing visual pictures of the bonds within molecules What it says: covalent bonds are formed by overlapping orbitals!!! H2, HF and F2 are good examples of how it works (lets draw them)

Sigma Bonds:  A covalent bond resulting from the formation of a molecular orbital by the end-to-end overlap of atomic orbitals, denoted by the symbol σ. *All single bonds are Sigma bonds

Pi bond: A covalent bond resulting from the formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane perpendicular to a line connecting the nuclei of the atoms, denoted by the symbol π. ** Double and Triple bonds create Pi bonds

Valence Bond Theory: Important points for VB Theory: 1. Orbitals overlap to form a bond 2. Two electrons fit in the overlapping space. Usually each atom supplies 1 electron 3. Both electrons are simultaneously attracted to both nuclei, and are hence most likely to be found between the nuclei

The Problem: Simple orbital overlap doesn’t explain the VSEPR shapes of central atoms … For example, why is methane, CH4, tetrahedral? p orbitals tetrahedral It seems as though the p orbitals do not work for trig. planar, tetrathedral, etc

Hybridization:

4 domain hybridization: Example: C in CH4 Atomic Orbitals of C: Hybrid Orbitals:

3 domain hybridization: Example: B in BH3 Atomic Orbitals of B: Hybrid Orbitals:

2 domain hybridization: Example: Be in BeCl2 Atomic Orbitals of Be: Hybrid Orbitals:

What about expanded octets? There are only enough s and p orbitals to give us four equivalent hybrid orbitals (tetrahedral). What about when we need five or six? 2p 2p 2p 2s

What about expanded octets? There are only enough s and p orbitals to give us four equivalent hybrid orbitals (tetrahedral). What about when we need five or six? 3d 3d 3d 3d 3d 3p 3p 3p 3s For years it was taught that for larger atoms, d orbitals could be hybridized as well. Now, we aren’t so sure …

Hybrid orbitals video

EASIER WAY TO PREDICT HYBRIDIZATION: 1.) Draw Lewis Structure 2.) Count number of domains **Domains = # of atoms and lone pair of electrons 3.) Choose hybridization whose superscripts = domain Example: Try for Methane, CH4

Draw diagrams showing the overlapping orbitals in the following molecules. Label any sigma bonds. List which orbitals overlap to create each bond Be sure to draw hybrid orbitals for any central atoms that hybridize before bonding. Cl2 HCl C2H6

Hybridization: Describe the bonding in ethane, C2H6 - What is the hybridization of any central atoms? - What orbitals overlap to produce the C – H bond? - What orbitals overlap to produce the C – C bond? - Type of bond? - Draw it!!!