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Chemical Bonding and Molecular Structures

Understanding the forces that cause a group of atoms to behave as a unit and achieve the lowest possible energy through chemical bonding.

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Chemical Bonding and Molecular Structures

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  1. A.P. Chem. Chapter 8/9 (Forces that cause a group of atoms to behave a unit) (A system can achieve the lowest possible energy by behaving this way) Bond Energies Chemical Bonding (always positive) Type: IONIC POLAR COVALENT COVALENT DEN large moderate zero creates polar bonds Electronegativity: the ability to attract shared pairs of electrons Transfer of electrons Unequal Sharing of electrons Sharing of electrons creates ions Estimate DH DH=SB.E.react – SB.E.prod Size changes Processes: Especially Lattice Energy ENERGY CALCULATIONS Delocalized Electron Model Aliphatic Aromatic** Organic Molecules Derivatives ▲(Coulomb’s Law) Needed to explain aldehydes Formal Charge s Bond Order ketones Localized Electron Model magnetism Multiple Bonds Para alcohols Molecular Orbitals Dia amines p s Resonance ethers types Atomic Orbitals Probonding p, p* organic acids Octet Rule Antibonding esters s* Know the exceptions Dictates geometry sp EPA’s and Shapes sp2 VSEPR Lewis Structures sp3 hybridization …needed modification dsp3 ** follows Huckels Rule (ring structure with 4n+2 pi electrons) d2sp3

  2. On bright sunny days, you may notice the fresh-air smell of this gas if it’s present in small concentrations. It’s name, however derives from a Greek word that means “to smell” because the normal odor in higher concentrations is disagreeable. You may notice this odor around faulty electrical equipment. It is a faintly blue gas that’s used a water disinfectant and bleach (especially in Europe). On bright sunny days, you may notice the fresh-air smell of this gas if it’s present in small concentrations. It’s name, however derives from a Greek word that means “to smell” because the normal odor in higher concentrations is disagreeable. You may notice this odor around faulty electrical equipment. It is a faintly blue gas that’s used a water disinfectant and bleach (especially in Europe). The gas is stable except at elevated temps or in the presence of a catalyst where it decomposes. Unfortunately, this happens to our dismay in the upper levels of our atmosphere where it’s very much needed… O3 (OZONE) Wild Bill Kernion

  3. O3 18 ← # of valence electrons = ______ 2-D Lewis diagram ↓ 3-D Lewis diagram ↓ # of bonds around the central oxygen = ______ 2 O O O # of lone pairs around the central oxygen = ______ 1 Trig. Planar EPA = ______________ Bent Shape = _____________ Are the bonds polar? Y or N O O O Hybridization on right side oxygen = _______ sp2 Bond angle about the central oxygen ______ <120° Total number of sigma bonds in molecule = ________ 2 Total number of pi bonds in molecule = ________ 1 The valence configuration of oxygen the atom: _______________________ 2s2 2px2 2py1 2pz1 (3) sp2 hybrids

  4. To get a better view we separate out those electrons that appear to be in different locations in different resonance forms and use MO theory to describe their behavior. The other electrons are described using the localized electron model sp2 hybridization Uses the molecular orbital model Uses the localized electron model

  5. O→ 2s22px22py12pz1 unhybridized p orbital

  6. molecule. Pi MO’s: Click here for Rotating Pi Orbitals of O3 Notice: only the bonding and non-bonding pi orbitals are occupied. We have only four electrons in the pi system and we fill up the possible pi energy levels in the same way that we do atoms, i.e. start at the lowest and use Pauli exclusion principle and Hund’s rule. The total PI bond order is (2 - 0)/2 = 1 but since the bonding orbital is spread over two O-O bonds the pi bond order per O-O bond is ½ = 0.5 . The O-O sigma bonds contributes a sigma bond order of 1.0 to each O-O bond so the TOTAL bond order per O-O bond is 1.0 + 0.5 = 1.5 which again predicts the O-O bond is half way between O-O and O=O. The pi electrons in the non-bonding orbital spend half their time on one end oxygen and half their time on the other end oxygen. Adding in the sigma non-bonding pairs we get an average of 2 ½ non-bonding pairs per end O atom as was predicted by averaging the resonance forms.

  7. Benzene 6 unhybrized p electrons (1 from each carbon) 6 molecular orbitals

  8. HCOOH 18 The organic name of this molecule is _________________________ ← # of valence electrons = ______ methanoic acid 2-D Lewis diagram ↓ 3-D Lewis diagram ↓ O # of bonds around the central carbon = ______ 3 # of lone pairs around the central carbon = ______ 0 Trig. Planar EPA = ______________ H C O Trig. Planar Shape = _____________ Are the bonds polar? Y or N H Hybridization on central carbon = __________ sp2 Bond angle about the carbon ______ 120° Total number of sigma bonds in molecule = ________ 4 Bond angle about oxygen 2 ________ 104.5° Total number of pi bonds in molecule = ________ 1 The valence configuration of carbon the atom: _______________________ 2s2 2px1 2py1 The excited configuration of carbon the atom (prior to hyb.): _______________________ 2s1 2px1 2py1 2pz1 (3) sp2 hybrids

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