1Tentative content material to be covered for Exam 2 (Wednesday, November 2, 2005)Chapter 16 Quantum Mechanics and the Hydrogen Atom16.1 Waves and Light16.2 Paradoxes in Classical Physics16.3 Planck, Einstein, and Bohr16.4 Waves, Particles, and the Schrödinger Equation16.5 The Hydrogen AtomChapter 17 Many-Electron Atoms and Chemical Bonding17.1 Many-Electron Atoms and the Periodic Table17.2 Experimental Measures of Orbital Energies17.3 Sizes of Atoms and Ions17.4 Properties of the Chemical Bond17.5 Ionic and Covalent Bonds17.6 Oxidation States and Chemical BondingChapter 18 Molecular Orbitals, Spectroscopy, and Chemical Bonding18.1 Diatomic Molecules18.2 Polyatomic Molecules18.3 The Conjugation of Bonds and Resonance Structures18.4 The Interaction of Light with Molecules18.5 Atmospheric Chemistry and Air Pollution.2Today’s lecture will be combination ppt and “chalk”lecture on how to create molecular orbitalconfigurations of electrons by the appropriatecombination of atomic orbitals.First there will be a ppt review of the key ideas ofChapter 17 on how atomic electron configurations canbe employed to understand the properties ofelementsFollowed by a chalk talk introduction to molecularorbital theory3The chemical behavior of atomsAn atom’s chemical behavior depends strongly onhow many valence electrons it has and one theelectronic configuration of the valence electrons.For the representative elements, the key valenceelectrons are the ns and np electrons which build upto a final core ns2np6 noble gas configuration.4Closed shells, core electrons and effective nuclear chargeElectrons in closed shells are inert because they are“buried” close to the nucleusClosed shells are “core” electrons of an atombecause they “screen” electrons in the “outer”orbitals outside the closes shell from the nuclearcharge ZThe electrons in the outer orbitals see an effectivenuclear charge, Zeff, not the full nuclear charge, ZThe outer electrons are the valence electrons5En = -(Zeff2/n2)Ry rn = (n2/Zeff)a0En ~ -1/rnThe Bohr one electron atom as a starting point for theelectron configurations of multielectron atoms.Key ideas:(1) Larger Zeff more energy required (IE) to remove e-(2) Smaller r more energy required (IE) to remove e-(3) Larger Zeff more energy gained (EA) when adding a e-(4) Smaller r more energy gained (EA) when adding a e-Replace Z (actual charge) with Zeff (effective charge)6Z is roughlythe same downa column:r, n increasesIE decreasesKey formulae are derived from the one electron Bohr atom:n is roughly the same along a periodr ~ n/ZE ~ Z/n E ~ 1/r IE increases, r decreasesZ increases:7Orbital energies from photoelectron spectroscopyNote the steeper slope of the drop of increasing stability with the 3dand 4f orbitals:4s3d4pNote the big dropin the energy ofthe 3d orbitalsstarting atZ = 30 for Zn:4s2d10This energy dropleaves the 4s and4p as the valenceelectrons8Chapter 18 Molecular orbitals and spectroscopy18.1 Diatomic molecules18.2 Polyatomic molecules18.3 Conjugation of bonds and resonance structures18.4 The interaction of light and matter (spectroscopy)18.5 Buckyballs918.1 Diatomic moleculesConstructing molecular orbitals from atomic orbitalsOrbital correlation diagramsHomonuclear and heteronuclear diatomic moleculesConstructive and destructive interference of wavesBonding and anti-bonding molecular orbitalsMO energies, AO parentage, Bond orderDiamagnetism of N2 and paramagnetism of O210Molecular Orbitals and Diatomic MoleculesMolecular orbitals: orbitals that span two or more atoms.Atomic orbitals: orbitals that are localized on single atoms.Constructing molecular orbitals (MOs) by overlappingatomic orbitals (AOs)! bonds: electron density of MO directed along bond axis" bonds: electron density of MO has a nodal plane thatcontains the bond axis11H2 is more stable than 2 H atoms. Why?Quantum mechanics and molecular orbital theory provide anexplanation: the overlap of atomic orbitals (waves)12Constructing MOsThe overlap Two AOs is treated as the overlap andinterference of two wavesIf the waves are in phase the interference isconstructive and add to give a larger total amplitudeIf the waves are out of phase the interference isdestructive and add to cancel a smaller total amplitudeor a zero amplitude (node)13An electron between two nuclei pulls the nucleitogether and is bonding.An electron beyond two nuclei pulls the nuclei apartand is anti-bonding.14A signature property of waves in the phenomena ofconstructive and destructive interference15Constructive and destructive interference ofwaves:constructive (bonding), destructive (anti-bonding)1s orbitals: wave interference16Constructive (top) and destructive (bottom) interferenceof two 1s orbitalsAn electron in a !1sorbital has anenhancedprobability of beingfound between thenuclei.An electron in a ! #1sOrbital has a reducedprobability of beingfound between thenuclei.17Rules for constructing ground state electronicconfigurations of homonuclear diatomic orbitalsCombine AOs to generate a set of molecular orbital fromconstructive and destructive interference of the AOsThe number of final MO’s must equal the number ofcombined AO’sOrder the MO’s by energy from lowest to highest (in amanner analogous to the procedure for AO’sPut the available electrons in the MO’s following theAufbau principle, the Pauli principle and Hund’s rule18Why are molecules more stable than the separatedatoms?Because there is more bonding than anti-bondingHow can you predict the stability of simple molecularspecies?Which of the following are stable or unstable?H2, H2+, He2, He2+Through orbital correlation diagrams19Correlation diagram for the overlap of two 1s orbitalsThe electronic configuration of a H2 molecule is !1s2The subscript (1s) tells which AOs are combined, thesuperscript (2) tells how many electrons are in the MO20A shared pair of electrons make a single covalent bondElectrons in bonding orbitals enhance bonding,electrons in anti-bonding orbitals reduce bondingBond order is a measure of the bonding between twoatoms: 1/2[(e in bonding MOs) - [(e in anti-bondingMOs)]21What is the bond order of the first electronicallyexcited state of H2?The electronic configuration of the first excitedstate of H2 is (!1s)1(! *1s)1.Bond order = 1/2(1 - 1) = 0Photochemical excitation of H2 makes it fly apartinto 2 H