Chem 150 week 10 Handout 3 MO Theory • Molecular Orbital Theory: Molecular orbitals, like atomic orbitals, are wave functions. Combination of atomic orbitals results in constructive and destructive interference of the wavefunctions. Constructive interference addition of wavefunctions Destructive interference subtraction of wavefunctions Consider the hydrogen molecule: from Petrucci Ideas and Rules about Molecular Orbitals: 1. The number of molecular orbitals (MOs) formed is equal to the number of atomic orbitals combined. 2. The bonding MO is at lower energy than the original atomic orbitals, and the antibonding MO is at higher energy. 3. In the ground-state configuration, electrons enter the lowest energy MOs first. 4. The maximum number of electrons in a given MO is two (Pauli exclusion principle). 5. In ground-state configurations, electrons enter MOs of identical energies singly before they pair up (Hund’s rule). 6. Bond order = (# of electrons in bonding MOs - # of electrons in antibonding MOs)/2 Diatomic Molecules of the First-period Elements from Petrucci Molecular orbitals by combining p orbitals Combining two 2px orbitals results in a σ*2p antibonding orbital and a σ2p bonding orbital Combination of the 2py and 2pz orbitals results in two π*2p antibonding and two π2p bonding orbital s* 2p C2 p* 2p p* 2p 2p 2p s2p p2p p2p s* 2s 2s 2s s2s s* 1s 1s 1s s1s s* 2p C2 p* 2p p* 2p 2p 2p s2p p2p 2s p2p s* 2s 2s s2s 1s s* 1s s1s 1s s* 2p O2 p* 2p p* 2p 2p 2p p2p p2p s2p s* 2s 2s 2s s2s 1s s* 1s 1s s1s s* 2p O2 p* 2p p* 2p 2p 2p p2p p2p s2p 2s s* 2s 2s s2s 1s s* 1s s1s 1s
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