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Chapter 9 Bonding and Molecular Structure: Orbital Hybridization and Molecular Orbitals

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1Chapter 9Bonding and Molecular Structure: Orbital Hybridization and Molecular OrbitalsHomework Chapter 9– 11, 21, 25, 27, 29, 31, 35, 39, 45, 51, 65Chapter 9 Broken into two different sections discussing two different theories on molecular bonding.– Valence bond theory– Molecular orbital theoryVALENCE BOND THEORY VALENCE BOND THEORY — LinusPauling valence electrons are localized between atoms (or are lone pairs). half-filled atomic orbitals overlap to form bonds.Valence Bond Theory A model of bonding in which a bond arises from the overlap of atomic orbitals on two atoms to give a bonding orbital with electrons localized between the atoms. Helps explain the molecular shapes. Takes the different orbital types (s, p, d) and creates hybrids of them.Sigma Bond Formation by Orbital OverlapTwo s orbitals overlap2Sigma Bond FormationTwo s orbitals overlapTwo p orbitals overlapTypes of Bonds Two basic types of bonds.–Sigma (σ) bonds. A bond formed by the overlap of orbitals head to head, and with bonding electron density concentrated along the axis of the bond. Can think of it as a single bond.–Pi (π) bonds. The second (and third, if present) bond in a multiple bond; results from sideways overlap of p atomic orbitals.Components of our known bonds.– Single bond = 1 sigma bond– Double bond = 1 sigma bond + 1 pi bond– Triple bond = 1 sigma bond + 2 pi bondsUsing VB TheoryBonding in BF3planar triangleangle = 120oFFFBoron configuration↑↑↓↑↓2p2s1s••••••••••••••••••BBonding in BF3 How to account for 3 bonds 120oapart using a spherical s orbital and p orbitals that are 90oapart? Pauling said to modify VB approach with ORBITAL HYBRIDIZATION — mix available orbitals to form a new set of orbitals — HYBRID ORBITALS — that will give the maximum overlap in the correct geometry.Bonding in BF3rearrange electronshydridize orbs.unused porbitalthree sp2hybrid orbitals2p2s• The three hybrid orbitals are made from 1 s orbital and 2 p orbitals → 3 sp2hybrids.Bonding in BF3• Now we have 3, half-filled HYBRID orbitals that can be used to form B-F sigma bonds.3An orbital from each F overlaps one of the sp2hybrids to form a B-F σ bond.Bonding in BF3↑↑BFFF↑↑↑BFFF↑↑↑↑BFFF↑↑↑BFFF↑↑Bonding in CH4How do we account for 4 C—H sigma bonds 109oapart? Need to use 4 atomic orbitals — s, px, py, and pz—to form 4 new hybrid orbitals pointing in the correct direction.109o109o4 C atom orbitals hybridize to form four equivalent sp3hybrid atomic orbitals.Bonding in a Tetrahedron —Formation of Hybrid Atomic OrbitalsBonding in a Tetrahedron —Formation of Hybrid Atomic Orbitals4 C atom orbitals hybridize to form four equivalent sp3hybrid atomic orbitals.Bonding in CH4The Hybridization of the Orbitals CH4– How many bonds off of the carbon center? 4– What is the noble gas notation of carbon? [He] 2s2, 2p2– So we can take the s and p orbitals of the carbon and create hybrid orbitals.4sp3Hybridization for carbon in CH4Energy2s2p orbitalsFour sp3hybrid orbitalsElectrons available to form σ bondsGiving us the ability to form 4 σ bonds.Also Works for Other sp3Hybrid Compounds NH3– Nitrogen has 3 bonds off of it and a lone pair of electrons. H2O– Oxygen has 2 bonds off of it and two lone pair of electron.Four sp3hybrid orbitalsFour sp3hybrid orbitalsHow to Determine the Hybridization of an Atom1. Draw the Lewis Structure if not given.2. Pick a specific atom if one it not specified.3. Count up the number of σ bonds off of it.4. Count up the number of lone pairs of electrons around it.5. Add these two numbers up.6. Use the s, p, d orbitals (IN THAT ORDER) to equal the number in part 5. Remember there is only one s orbital, three p orbitals and fived orbitals.For Example(all bonds are single bonds) An atom with 4 bonds off of it.–sp3hybridized An atom with 3 bonds and one lone electron pair.–sp3hybridized An atom with 1 bond off of it.– s hybridized (which is really not a hybridization) An atom with 2 bonds off of it.– sp hybridized An atom with 2 bonds and one lone electron pair.–sp2hybridized An atom with 5 bonds and one lone electron pair.–sp3d2hybridized An atom with 5 bond off of it.–sp3d hybridizedMultiple Bonds C2H4– Draw the Lewis Structure.– What is the hybridization around carbon? sp25Hybridization Around Carbon in C2H4Energy2s2p orbitalsThree sp2hybrid orbitalsUn-hybridized p orbitalGiving us the ability to form 3 σ bonds and 1 π bond.σ and π Bonding in C2H4σ and π Bonding in CH2O σ and π Bonding in C2H2More Examples H2CO C2H2 CSe2 NH4+Exploring O2 Lewis Dot Structure Determine the Hybridization around the oxygens–How many σ bonds? 1– How many lone pairs? 2– Therefore we need how many orbitals? 3– What is the hybridization? sp26Hybridization Around Oxygen in O2Energy2s2p orbitalsThree sp2hybrid orbitalsUn-hybridized p orbitalGiving us the ability to form 1 σ bonds and 1 π bond.The Bonding in O2Three sp2hybrid orbitals of oxygenUn-hybridized p orbitalThree sp2hybrid orbitals of oxygenUn-hybridized p orbitalForms the σ bondForms the π bondRemember that since the bond between the oxygens is covalent the electrons are shared equally. This causes the two ½ empty orbitals to be filled. Therefore oxygen is ___magnetic.Paramagnetic or Diamagnetic? Based upon the valence bond theory what would you guess be about the magnetism of O2?Molecular Orbital Theory Molecular orbital (MO) theory is an alternative way to view orbitals in molecules. MO theory assumes that pure atomic orbitals of the atoms in the molecule combine to produce orbitals that are spread out, or delocalized, over several atoms or even an entire molecule.The Basic Principles of Molecular Orbital Theory1. The total number of molecular orbitals is always equal to the total number of atomic orbitals contributed by the atom that have combined.2. The bonding molecular orbital is lower in energy than the parent orbitals, and the anti-bonding orbital is higher in energy.3. The electrons of the molecule are assigned to orbitals of successively higher energy according to the Pauli Exclusion Principle and Hund’s Rule.4. Atomic orbitals combine to form molecular orbitals most effectively when the atomic orbitals are of similar energy1stPrinciple of MO TheoryExample of a molecular


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