13_MOTheory - Bonding in molecules and solids Where we're...

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Unformatted text preview: Bonding in molecules and solids Where we're heading: The chemistry of modern materials (solids), a=er a brief look at some key aspects of bonding in molecules (molecular orbitals) How are atoms held together in molecules and solids? How does this contribute to the proper7es of a molecule or solid? Magne7sm (unpaired vs. paired electrons) Electrical (metals, semiconductors, insulators) Mechanical (hardness, strength) Thermal (mel7ng / boiling / decomposi7on) Bonding in Materials 1 Lewis dot structure for O2 Are the electrons paired or unpaired? Based on this structure: would you predict O2 to be paramagne7c or diamagne7c ? Frac7onal Dis7lla7on of O2 from air Air contains (by volume): 78.08% N2 (BP = 77 K) 0.93% Ar 20.95% O2 (BP = 90 K) 0.038% CO2 What is happening in the experiment? What does this tell us about the Lewis dot structure of O2? Bonding in Materials 2 Review of Covalent Bonding Orbital probability of finding an electron Energy of electron (energy needed to remove an electron from the orbital) Each orbital holds 2 electrons Bonding in Materials 3 Valence Bond Theory Bonding = increased electron density between nuclei = overlap of orbitals -bond: -bond: Bonding in Materials 4 Hybrid orbitals Mix atomic orbitals from the same atom to get hybrid orbitals. E.g. 1s + 3p = ________ hybrid orbitals Bonding in Materials Molecular Orbital (MO) Theory We are going to "create" molecular orbitals like hybrid orbitals mix atomic orbitals on different atoms to get Molecular Orbitals. Molecular orbitals are spread out OVER THE MOLECULE. Bonding in Materials Genera7ng molecular orbitals How do we generate molecular orbitals from atomic orbitals? Combine their wavefunc7ons. Destruc7ve interference (subtract) Construc7ve interference (add) two atomic wavefunc7ons "in", two molecular wavefunc7ons "out" Bonding in Materials 7 MO Theory: Energy levels What we learned for atomic orbitals s7ll holds: Fill the orbitals from low to high energy, half-filling each degenerate orbital first before pairing electrons in the same orbital Bonding in Materials 8 Bond Order of MO's: The bond order is defined as: [# bonding e # an7bonding e]= (BE AE) What is the bond order for H2? What is the bond order for He2? What does this physically mean? Bonding in Materials 9 MO Theory: Mixing of p-orbitals The p atomic orbitals can mix either "end-on" or "sideways" to create molecular orbitals Bonding in Materials 10 Second Row Diatomics The resul7ng MO diagram looks like this. There are both and bonding molecular orbitals and * and * an7bonding molecular orbitals. Bonding in Materials MO Theory The smaller p-block elements in the second period have a sizeable interac7on between the s and p orbitals. This flips the order of the and molecular orbitals in these elements. Bonding in Materials 12 Second-Row Diatomics MO Diagrams Bonding in Materials 13 How do we know this is true? Whether or not a molecule has paired or unpaired electrons is something that can be measured ! Paramagne7c: substance that is influence by a magne7c field (has unpaired electrons) Diamagne7c: substance with no unpaired electrons Bonding in Materials 14 How do we know this is true? Compare N2 and O2 in magne7c field What can MO theory tell us that Lewis dot structures can not? N2 Total # of valence e Bond order # unpaired e Bonding in Materials O2 Total # of valence e Bond order # unpaired e 15 MO Theory Con7nued. MO theory can be used to describe heteronuclear diatomic molecules & molecular ions such as: LiH (Lithium hydride), HF (Hydrogen fluoride), CO (Carbon monoxide) CN (Cyanide ion), NO+ (Nitrosonium ion) Valence bond theory 2 atomic orbitals overlap Molecular Orbital Theory MOs are single orbitals spread out over ENTIRE MOLECULE Bonding in Materials 17 Summary of molecular orbitals Number of MO's formed equals the number of AO's combined Each MO holds a maximum of two spin-paired electrons For degenerate MO's (same energy), one electron (same spin) in each MO before pairing (Hund's rule) AO's combine best with AO's of similar energy The effec7veness with which two atomic orbitals combine is propor7onal to their overlap (size, shape, energy) more overlap results in a larger spliong between the bonding (lower energy) and an7bonding (higher energy) MO's Bonding in Materials 18 ...
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This note was uploaded on 01/18/2012 for the course CHEM 112 taught by Professor Vandersluys,lorschmid,kylem during the Summer '07 term at Penn State.

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