CHE 349 Physical Chemistry for Life Sciences Lecture 20 MO Theory Homonuclear Diatomic Molecules together A homonuclear diatomic molecule is composed of two identical elements bonded As a result of increase in electronegativity and a decrease in atomic size the order of 2 Ex 2 2 and The will be reversed for O to Ne will move up and will move down 2 2 2 From Li2 to N2 From O2 to Ne2 22 Using a Molecular Orbital MO diagram write the electron configuration for the cation 1 Determine the of valence electrons Oxygen is in group 6A so it has six valence electrons and there are two of them so 6 2 12 minus two is equal to 10 total valence electrons for 22 2 Construct the Molecular Orbital diagram based on the location of the valence electrons a Period 1 elements start with 1s Period 2 elements start with 2s and Period 3 elements start with 3s and so on b Oxygen is located in Period 2 so it will start with 2s However since the standard molecular orbital diagram that starts with 2s won t have the capacity to contain all 10 valence electrons you ll actually start with 1s 3 Fill in the total number of valence electrons for each element into the molecular orbitals 4 Fill in the MO electron configuration 1 2 1 2 2 2 2 2 2 2 2 4 2 4 2 2 Hybridization Hybridization represents the idea of valence shell atomic orbitals mixing to form hybrid orbitals In order to form more bonds and increase stability elements must hybridize their atomic orbitals Hybridizatio n of electron groups Electron Geometry Linear Hybridization Hybridized Bond Angle Orbitals Unhybridized Orbitals Examples 180 Two p orbitals Trigonal planar One p orbital Trigonal planar 2 3 4 sp 2 3 Tetrahedral Zero p orbitals Tetrahedral For trigonal planar 120 For bent 120 For tetrahedral 109 5 For trigonal pyramidal 109 5 For bent 109 5 Bent Trigonal pyramidal Bent 3 5 Trigonal Bipyramidal Four d orbitals Trigonal bipyramidal For trigonal bipyramidal 120 equatorial 90 axial For seesaw 120 equatorial 90 axial For t shaped 90 For linear 180 For octahedral 90 For square pyramidal 90 For square planar 90 Seesaw T shaped Linear Square pyramidal Square planar 6 Octahedral Three d orbitals Octahedral 3 2 Quantum Numbers Nodes A node is the region within an atom where the probability of finding an electron is zero Electron shell is the region where an electron resides with the highest probability Total number of nodes n 1 Bonding vs Antibonding Molecular Orbital A bonding molecular orbital is the region of increasing electron density between elements An antibonding molecular orbital is a region with decreasing electron density node that that promotes bond formation prevents bond formation Bonding a k a additive molecular orbitals have lower energies than isolated atomic orbitals which leads to an increased electron density between 2 atoms and greater stability of the overall molecular structure Antibonding a k a subtractive molecular orbitals have higher energies than isolated atomic orbitals which leads to a decreased electron density between 2 atoms and less stability of the overall molecular structure Linear combination of atomic orbital LCAO The addition of the atomic orbitals of two atoms to make molecular orbitals is referred to as the linear combination of atomic orbitals is the symbol for the wave function also known as the movement of an atom in the form of a wave The molecular orbital with the higher energy gives rise to a repulsive state and is called as antibonding molecular orbital The molecular orbital with the lower energy gives rise to an attractive state and is called as bonding molecular orbital the wave function of electron of atom A the wave function of electron of atom B Wave function of bonding molecular orbital Wave function of bonding molecular orbital If you re combining two s atomic orbitals let s say 1s and 1s then this is what their bonding and antibonding orbitals will look like If you re combining two p atomic orbitals let s say bonding and antibonding orbitals will look like and then this is what their 2 2 combining any of the two other p atomic orbitals and what their bonding and antibonding orbitals will look like 2 2 2 or and 2 If you re then this is Sigma and pi bonds are types of molecular orbitals that result from the overlap of atomic orbitals in covalent bonding bond molecular orbitals REMEMBER bonds refer to a double bond and or triple bond in an organic molecule bond molecular orbitals refers to the side to side overlap of atomic orbitals This overlap occurs parallel to the internuclear axis bonds have one nodal plane perpendicular to the bond axis This means there is a region with zero electron density between the nuclei of the bonded atoms The presence of nodal planes in pi bonds affects the electron density distribution and the overall shape of the molecular orbital NOTE Electron density refers to the probability of finding an electron in a particular region of space around an atomic nucleus or within a molecule In other words it describes the likelihood of locating an electron at a specific point in an atom or a molecule bond molecular orbitals REMEMBER bonds refer to a single bond in an organic molecule bond molecular orbitals refers to the head to head overlap of atomic orbitals along the internuclear axis Sigma bonds are the strongest covalent bonds their superior strength and stability contributes to the direct and effective overlap of atomic orbitals The direct overlapping of atomic orbitals along the internuclear axis in sigma bonds allows for maximum orbital overlap leading to a more effective sharing of electrons between the bonded atoms Sigma bonds have no nodes along the bond axis The electron density is concentrated along the axis connecting the nuclei of the bonded atoms HOMO Highest Occupied Molecular Orbital The highest energy molecular orbital that is closed or fully occupied Electrons in the HOMO are involved in bonding LUMO Lowest Unoccupied Molecular Orbital The lowest energy molecular orbital that is vacant or unoccupied It s the next available orbital for electrons to enter after the HOMO is completely filled NOTE The HOMO LUMOs are referred to as frontier orbitals and can strongly influence the chemistry of the molecule since chemical reactions uncover changes of the electron density Delocalized Orbitals A delocalized orbital refers to an orbital that extends over more than two adjacent atoms In molecules or molecular ions delocalized orbitals are formed through the constructive