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NCSU CH 101 - Molecular Orbital Theory

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Lecture 14Outline of Last LectureI. Bond Order in Complex MoleculesII. Pi and Sigma BondsIII. Introduction to HybridizationOutline of Current LectureI. Molecular Orbital Theory (MO Theory)A. HOMO and LUMO.II. Bonding MO’sIII. Antibonding MO’s IV. Nonbonding MO’sA. Acceptable and Unacceptable MO bondingCurrent LectureI. Molecular Orbital Theory (MO Theory)- Molecular Orbital Theory shows us the different possible bonds between the electrons in a molecule (this is also known as delocalized bonding). MO theory tells us that when electrons combine, they could create a bonding orbital an antibonding orbital, or a nonbonding orbital.Paramagnetic-An element is paramagnetic if it has unpaired electronsDi-magnetic- If the element fills all of the orbitals, like Ne, it is di-magnetic.Bond Order- To find the bond order through the MO theory you take the number of electrons that are bonding, subtract the number of electrons that are antibonding, then divide by two.Here is the MO diagram (some call it a staircase) you will use to determine the bond order, if the element is paramagnetic/di-magnetic, and where the LUMO and HOMO is. The sigma and pi bonds with a star next to them represent antibonding.CH 101 1st EditionA. HOMO and LUMOHOMO- The acronym homo stands for highest occupied molecular orbital. If you visualize an electron configuration chart, the HOMO is the last orbital that is fully occupied with electrons. LUMO- This stands for lowest unoccupied molecular orbital. This orbital would be the orbital that has no electrons in it, completely unoccupiedEx) Fill out the MO diagram for O2. Determine the HOMO and LUMO, the bond order, and decide if it is paramagnetic or diamagnetic.Valence electrons for O2 = 12 <- First look at the periodic table and determine the valence electrons(remember it is O2 so the valence electrons are doubled.)<- Take the 12 valence electrons and distribute them into the orbitals of your MO diagram<- Because there are 2 lone pairs, O2 is paramagnetic.Bonding electrons = 8Antibonding electrons = 4(8-4)/2 = 2 therefore the bond order is 2.II. Bonding MO’s- Also called constructive MO’s, bonding MO’s occur when the electron waves are the same so when they come together they form a bond. These bonds form molecular orbitals by combining in phase through constructive overlap. In pictures, combining electrons that are the same color are going to bond.and and <-Sigma Bondingand <- Pi bondingIII. Antibonding MO’s- Antibonding or deconstructive bonding occurs when electrons with opposite spins meet and the electron waves cancel each other out, think of it like the graphs of sin and cos. Antibonding creates nodal planes between electrons that are combined out of phase. These bonds are higher in energy than the bonding MO’s. In pictures, antibonding occurs when electrons of different colors combine. A nodal plane forms between the electrons that are antibonding, hence the number of antibonding lectrons is equal to the number of nodal planes.and <- Pi antibondingand <- Sigma antibondingIV. Nonbonding MO’s- Nonbonding MO’s occur when electrons bond because they are separated by a node. In nonbonding, there is no energy change. .A. Acceptable and Unacceptable MO bonding.- It is important to understand what types of MO bonding is allowed, it all depends on the placementof the nodes. Here are the accepted bonds from lowest to greatest energy (the energy increases with the number of nodes). The black dots in the pictures count as a node because those electrons are nonbonding, and the area between each antibonding electron pair is also a node.Sigma Bonds4 nodes (high energy)3 nodes2 nodes1 node0 nodes (low energy)Pi Bonds4 nodes (high energy)3 nodes2 nodes1 node0 nodes (low energy)- There are new arrangements of electron orbitals as the amount of nodes increases but, if you are asked to identify an MO that is not correct, check the number of nodes and see if it matches these pictures


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