chemtest2studyguide - 1 MOLECULAR ORBITALS SOLUTIONS...

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1 MOLECULAR ORBITALS, SOLUTIONS, CHEMICAL KINETICS Exam II Study Guide Valence Bond Theory CANNOT explain Resonance Paramagnetic and diamagnetic Semiconductors Bond Order o The number of bonds between 2 atoms Molecular Orbital (MO) Theory Atomic Orbitals come together to form new orbitals that belong to the molecule as a whole o Atomic Orbitals (AO’s) Molecular Orbitals (MO’s) Orbital conservation: the same number of molecular orbitals are produced from the original number of atomic orbitals o n AO’s n MO’s Bonding is constructive (adding together the orbitals) and the new orbitals are LOWER in energy than the original AO’s o Electrons in bonding orbitals contribute to bonding between atoms o n/2 corresponding bonding orbitals Antibonding (denoted with a *) is destructive (subtracting orbitals) and the new orbitals are HIGHER in energy than the original AO’s o Electrons in antibonding take away from the bonding between two atoms o n/2 corresponding anti-bonding orbitals LUMO and HOMO Have a large say in reactivity and color LUMO - Lowest unoccupied molecular orbital o Can accept electrons from a filled orbital on another atom HOMO - highest occupied molecular orbital o Can donate electrons to an empty orbital on another atom Understanding A Molecular Orbital Diagram Valence orbitals: s, p x , p y , p z Rank p orbitals by the number of nodes o More nodes, higher energy, less stable o EX (refer to diagram below): π has 1 node, σ has 2 nodes, π * has 2 nodes, and σ * has 3 nodes Degenerate (same energy) AO’s combine to form the new MO’s Hund’s Rule and Pauli Exclusion Principle are followed Bond Order = (1/2) (# e-s in bonding) – (#e-s in anti-bonding)
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2 EX of a MO Diagram: N 2 (1s 2 2s 2 2p 5 ) Conductivity, Semiconductors, and Insulators Can all be explained using Molecular Orbital Theory 1 mole of H 1 mole AO’s 1 mole MO’s 1 mole of MO’s can hold 2 moles of electrons (2 e-s per orbital) π * σ E E σ * s s p p p p p p σ σ * π * π π AOs AOs MOs QuickTime  and a TIFF (Uncompressed) decompressor are needed to see this picture. P x P x P y P y Note: P z is the same as P y rotated 90 ° Antibonding (3 Nodes) P x - P x Bonding (2 Nodes) P x + P x Antibonding (2 Nodes) P y – P y Bonding (1 Node) P y + P y QUICK REVIEW σ - Cylindrically symmetrical π - Not cylindrically symmetrical Hund’s Rule - orbitals fill up singly before an orbital is doubly occupied Pauli Exclusion Principle - no two electrons can have the same set of quantum numbers (MO application: 2 electrons in an orbital must have different spins) HOMO : σ LUMO : π * NOTE : The extra protons in some atoms causes a switch in energy between σ 2p and π 2p (EX: O 2 , F 2 , and Ne 2 ) Bonding and Antibonding with P-Orbitals
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3 Insulator Covalent bonding is much stronger than orbital overlap (EX: Diamond) Wider separation between valence band (bonding) and conduction band (antibonding)
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