Class Notes


We were inspired by Dr Matthew Stoltzfus’s approach in flipped classrooms, here at KFUPM we adapted similar approach. Dr. Fus thankfully agreed to share his video lectures with us.

9.1 – 9.2 Molecular Shapes and VSEPR

When we draw molecules using the VSEPR theory, we first must identify the central atom, determine its valence electrons, then look at how many electrons the central atom will share with its surrounding atoms, or ligands. The videos shown below considers central atoms with either H, a halogen, or an OH group as ligands. For each molecule we will determine its molecular geometry, electron domain geometry, polarity, and the hybridization on the central atom.

9.2 – 9.3 AXn (X = H, X, OH) VSEPR Shapes and Polarity Part 1

9.2 – 9.3 AXn (X = H, X, OH) VSEPR Shapes and Polarity Part 2

9.2 – 9.3 AXn (X = H, X, OH) VSEPR Shapes and Polarity Part 3

We continue with VSEPR theory, as the videos shown below considers central atoms with either an O or an S as ligands. For each molecule we will determine its molecular geometry, electron domain geometry, polarity, and the hybridization on the central atom.

9.2 – 9.3 AXn (X = O, S) VSEPR Shapes and Polarity

We continue with VSEPR theory, as the video shown below considers central atoms with either an O or an S as ligands where a double bond is required to reach an octet. When a double bond is required to reach an octet, various structure can result with different energies. If molecules have different energies we cannot consider them to be resonance structures. We will use formal charges to determine which structure is the best one.

9.2 – 9.3 AXn (X = O, S db required) VSEPR Shapes and Polarity Part 1

9.2 – 9.3 AXn (X = O, S db required) VSEPR Shapes and Polarity Part 2

9.2 – 9.3 AXn (X = O, S db required) VSEPR Shapes and Polarity Part 3

When drawing Lewis Structures to represent shapes of molecules, you may have to make a choice between satisfying the Octet Rule or Using Formal Charges to best represent each molecule. There is still debate between researchers and the following video explains why this is the case.

9.2 – 9.3 Formal Charge vs. The Octet Rule

Another scenario we encounter in VSEPR theory is a central atoms with both H, X, or OH and an O or an S as ligands. We will also use the rules from the previous video to predict the actual shape of the molecule.

9.2 – 9.3 AXn (X = both H, X, OH and O,S) VSEPR Shapes and Polarity

The last scenario we encounter in VSEPR theory is oxyacids. Once we rewrite each formula in a more convenient way, we can use all the rules from the previous videos to predict the actual shape of oxyacids.

9.2 – 9.3 AXn VSEPR Shapes of Oxyacids

VSEPR Theory allows us to predict the shapes, bond angles, and polarity of a molecule, but it really doesn’t tell us why bonds form between atoms. A better understanding of Quantum Mechanics will allow us to understand why bonds form they way they do.

9.4 Quantum Mechanics and Chemical Bonding

In order to explain the reasons behind the observations in VSEPR Theory, valence bond theory was developed. Valence bond theory, which utilizes the overlap of atomic orbitals to explain bonding.

9.4 Orbital Overlap in Covalent Bonding

We have mentioned hybridization of the central atom before, but we really haven’t explained it much. Quantum Mechanics allows us to treat atomic orbitals as mathematical functions. Just as mathematical functions can be added or subtracted from one another, atomic orbitals can be “added or subtracted” from another. We refer to this phenomenon as “mixing.” The atomic orbitals on an atom can mix together to form hybrid orbitals.

9.5 Hybrid Orbitals

Depending on how the atomic orbitals on a central atom hybridize, there may be an additional orbital available for bonding. If this is the case, these additional orbitals can overlap, and pi bonding can occur.

9.6 Multiple Bonds

Even though Valence Bond Theory does a better job of explaining bonding than VSEPR Theory, there are a few physical properties it cannot explain, such as magnetism. In order to appropriately address properties such as magnetism, Molecular Orbital Theory was developed.

9.7 Molecular Orbital Theory

When atomic orbitals overlap to form molecular orbitals their energies change. A Molecular Orbital Diagram is a convenient way to arrange the energies of the molecular orbitals created from the atomic orbitals so that we can appropriately populate these molecular orbitals with electrons. By properly filling the electrons in a Molecular Orbital diagram the bond order and magnetic properties of a molecule can be determined.

9.7 Molecular Orbital Diagrams

For Molecular Orbital Diagrams in the 2nd period of the periodic table, we must consider how the p orbitals interact, or mix with, the s orbitals. This will have an impact on the resulting molecular orbital diagrams.

9.8 2nd Row Diatomic Molecular Orbital Diagrams