CHEM 2211 1st Edition Lecture 1 Outline of Current Lecture I II Molecular Orbital Theory MO Linear Conservation of Atomic Orbitals LCAO Vocabulary Atomic Orbital Sigma bonds Molecular Orbital Electron Configuration Constructive Overlap Sigma Star Destructive Overlap Electronegativity electronegative Nodes nodal Electron Density Antibonding Current Lecture I Molecular Orbital Theory MO A Atomic orbitals are the halo around an atom s nucleus where electrons are most likely going to be B Molecular Orbitals are the combination of atomic orbitals C MO s add both constructively and destructively i Constructive overlap occurs when two atoms in the same phase add together to create a large molecular orbit ii Destructive overlap occurs when two atoms out of phase try to bond When they contact each atom s electron try to move as far away as possible creating a Nodal space in between where no electrons are present This effect is called Antibonding Figure 1 As Hydrogen atoms begin to bond their atomic orbitals begin to fuse into one molecular orbital This is an example of Constrictive overlap Figure 2 When out of phase Hydrogen atoms try to bond they create a nodal space where no electrons exist This is called Destructive Overlap and is characteristic of Antibonding II Linear Conservation of Atomic Orbitals A In order to a count for all the electrons the number of atomic orbitals consumed is equal to the number of molecular orbitals formed 2 Atomic Orbitals 2 Atomic Orbitals 4 Molecular Orbitals B Recall that Hydrogen atoms form sigma bonds when bonded AND have a 1s1 electron configuration C As seen figure three the 2 individual Hydrogen atoms on the left and right combine their electrons to create a sigma bond at the bottom i The sigma star orbital at the represent the Antibonding orbital and is filled when the bond between the Hydrogens is broken Figure 3 D Let s see the same phenomenon in the pi bonds of ethylene Figure 4 Ethylene E The only change between the carbons bonding and the hydrogens bonding is the presence of the double pi bond F What happens when one molecule in a bond is more electronegative than the other as seen in the carbon oxygen bond of the carbonyl functional group Figure 5 Carbonyl Functional Group G Because Oxygen is more electronegative than Carbon it has a higher electron density This density causes the oxygen to be more similar to the sigma bond than the carbon Conversely because the carbon has fewer electrons around it it is closer in similarity to the empty pi star orbital Figure 6
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