LAST TIME 9Functional groups 9Electronegativity bond dipole moments polarity SELF STUDY see synopsis Lewis structures formal charge recap using 9 your general chemistry book 9Problems from Brown Foote 9Weeks s Pushing Electrons 9Tutorials on the ChemOffice Web and Vining CD s Lewis Structures In neutral molecules with no formal charges Element H valence e1 bonds 1 unshared e pairs 0 C 4 4 0 N 5 3 1 O 6 2 2 F Cl Br I 7 1 3 Fewer bonds More bonds negative formal charge positive formal charge Today 9VSEPR recap 9Resonance 9Molecular orbital theory 9polarity and nucleophiles electrophiles Resonance For many molecules and ions no single Lewis structure provides a truly accurate representation O O CH3 C and O CH3 C O Ethanoate ion Acetate ion the conjugate base of acetic acid Resonance Linus Pauling 1930s many molecules and ions best described by two or more Lewis structures contributing structures individual contributing structures related by writing a double headed resonance arrow molecule or ion is a hybrid of the contributing structures No flip flopping interconversion etc REALITY CHECK the contributing structures do not exist They are plausible arrangements which taken together represent reality Resonance Examples NO2nitrite ion O 2 equivalent contributing structures N O N O Ethanoate ion 2 equivalent contributing structures O CH3 C O O CH3 C O O Resonance All acceptable contributing structures must 1 have the same number of valence electrons 2 obey the rules of covalent bonding no more than 2 electrons in the valence shell of H no more than 8 electrons in valence shell of 2nd period elements 3rd period elements may have up to 12 electrons in their valence shells 3 differ only in distribution of valence electrons 4 have the same number of paired and unpaired electrons Resonance example CH3OCH2 Preference 1 structures with filled valence shells contribute more than those with unfilled valence shells CH 3 O C H H Greater contribution both carbon and oxygen have complete valence shells CH 3 O C H H Lesser contribution carbon has only 6 electrons in its valence shell Resonance CH3OCH2 continued same contributing structures Preference 2 structures with a greater number of covalent bonds contribute more than those with fewer covalent bonds CH 3 O C H H Greater contribution 8 covalent bonds CH 3 O C H H Lesser contribution 7 covalent bonds Resonance example CH3COCH3 acetone or dimethylketone Preference 3 structures with separation of unlike charges contribute less than those with noOcharge separation O CH3 C CH 3 Lesser contribution separation of unlike charges CH 3 C CH 3 Greater contribution no separation of unlike charges Resonance CH3COCH3 continued same contributing structures Preference 4 structures that carry a negative charge on the more electronegative atom contribute more than those with the negative charge on the less electronegative atom Lesser contribution C C H3 C CH3 C H3 C O O O CH3 Greater contribution H3 C CH3 Can be ignored Molecular Orbital Theory Electrons in atoms exist in atomic orbitals Electrons in molecules exist in molecular orbitals MO s Using Schr dinger equation we can calculate the shapes and energies of MO s Molecular Orbital Theory Rules combination of n atomic orbitals gives n MO s MO s are arranged in order of increasing energy MO s fill by same rules as for atomic orbitals Aufbau principle fill beginning with LUMO Lowest Unoccupied Molecular Orbital Pauli exclusion principle no more than 2 e in a MO Hund s rule filling of degenerate orbitals HOMO Highest Occupied Molecular Orbital Molecular Orbital Theory Terminology ground state lowest energy excited state NOT lowest energy sigma bonding MO sigma antibonding MO pi bonding MO pi antibonding MO Hybrid Orbitals z z Shapes of Atomic Orbitals s p d f y y Wave function may be x x positive or negative or zero at a nodal surfacean s orbital Hybridization of orbitals L Pauling a pz orbital the combination of two or more atomic orbitals to form a new set of atomic orbitals called hybrid atomic orbitals Hybrid Orbitals The Problem 2s and 2p atomic orbitals would give bond angles of approximately 90 contrary to observations for molecules like CH4 H2O HCCH instead we observe approximately 109 5 120 and 180 A Solution hybridization of atomic orbitals 2nd row elements use sp3 sp2 and sp hybrid orbitals for bonding Hybrid Orbitals We deal with three types of hybrid atomic orbitals sp3 1 s orbital 3 p orbitals sp2 1 s orbital 2 p orbitals sp 1 s orbital 1 p orbital Overlap of hybrid atomic orbitals can form two types of bonds depending on the geometry of the overlap bonds are formed by direct overlap bonds are formed by parallel overlay Overheads from Zumdahl hybrid orbitals