Theories of Chemical Bonding

Theories of Chemical BondingExam #4 (Chapter 8) on 7-DecemberOne 3 x 5 notecardChapter 8 OWL Deadline Tuesday NightChapter 9 OWL Deadline @ Final ExamFinal Exam (review session weekend before)Monday, 12-December @ 8AM in IRC 3 (here)~50 points just on Chapter 9 (about the same as the other chapters)~100 points cumulative (Chapters 2-8)One 81/2 x 11 note sheet allowedFinal Lab ExperimentThis Week (lets look @ WIKI Page)Laboratory Checkout Next Week!Whats Gotten Us Here.Electron ConfigurationsEnergy Level DiagramsValence ElectronsLewis StructuresIdentify Central Atom, Bonding and Lone PairsFormal ChargePeriodic TrendsSize, ElectronegativityBond PropertiesBond Energy, Length, Order, Polarity

Electron Pair GeometryClues to the shape of the molecule or ionMolecular GeometryDefinitive shape of the molecule or ionBond AnglesHowever, other than identifying ionic and covalent bonds, we havent really described how atoms bond together!Electrons in a covalent bond are shared, but how?

Lets Look at Boron TrifluorideB has 3 valence e-Fluorine has 7 valence e-We know that B follows an exception to the octet ruleLets look at the Lewis Structure

How do these 3 bonds form?We have a single s and three p orbitals on the boron atomWe have a single s and three p orbitals on the fluorine atomHow do they overlap to share electrons?Hybridization!

Valence Bond and Molecular Orbital TheoriesShare some principlesBonds are formed when electrons are shared between atomsThe sharing of electrons, and their attraction to the two nuclei of the atoms that are bonded lowers the total energy in the molecule or ionTwo types of bonds can formsigma () bonds which lie along the axis between to atoms in a molecule or ionAtoms can rotate around sigma bondspi () bonds which lie in regions outside of the axis between two atoms (but parallel to the axis)Atoms are fixed and cant rotateMultiple bonding6Valence Bond TheoryTakes into accountLewis structure (# of bonds)VSEPR (overall shape of the molecule or ion)The available orbitals that can be used for bondingQuantum MechanicsSpectroscopy results (like the colorimetry you used in lab)Thermodynamic dataHelps decide what types of hybrids will form

Hybridization (very systematic)To obtain the bonding description about any atom in a molecule:1.Write the Lewis electron-dot formula.2.Use VSEPR to determine the electron arrangement about the atom.3.From the arrangement, deduce the hybrid orbitals.4.Assign the valence electrons to the hybrid orbitals one at a time, pairing only when necessary.5.Form bonds by overlapping singly occupied hybrid orbitals with singly occupied orbitals of another atom.Some Tenets of HybridizationThe total number of orbitals you started with (all valence) MUST equal the number of orbitals that you end up withThe number of hybrid orbitals must equal the number of initial orbitals you started with (again all valence)Two electrons per orbitalThe number of sigma bonds equals the number of single bondsMultiple bonds are sigma + pi bondss, p, d orbitals are what we are working withValence ONLY!!!Lets Start with Methane (CH4)Write the valence electron configurationsCarbon has 4 valence electrons4 hydrogen atoms each have 1 valence e-Draw the Lewis structureEvaluate electron and molecular structureShape helps us understand hybridizationDetermine what orbitals will be combined (hybridized) to create the necessary bondsDescribe the hybridization

Lone Pairs ?Lone pairs of electrons can be placed in hybrid orbitals too.AmmoniaWrite electron configurations (valence)Lewis StructurePredict electron and molecular geometryDetermine what orbitals might combine and hybridize to describe the bondingConsider the best way to locate lone pairs (VSEPR)Describe the hybridizationLets do this on the board!

OK, Lets Get Back to BF3Lewis StructureElectron Pair and Molecular Structure (geometry)Predict HybridizationRemember-we are normally talking about hybridization about central atoms at this point.Describe Hybridization

3rd Period Elements (d orbitals)Hybrids can be made from combinations of s, p and d orbitals.Only available to elements in the 3rd period or higher (n = 2 only has s and p orbitals available)Same rules applyTotal number of orbitals are constantNumber of hybrid orbitals equals the number of orbitals combined to make themSome orbitals may be left unhybridized5 electron pairs (sp3d)

6 electron pairs (sp3d2)

How is this systematic ?

Hybrid orbitals are named by using the atomic orbitals that combined:one s orbital + one p orbital gives two sp orbitals one s orbital + two p orbitals gives three sp2 orbitals one s orbital + three p orbitals gives four sp3 orbitals one s orbital + three p orbitals + one d orbital gives five sp3d orbitals one s orbital + three p orbitals + two d orbitals gives six sp3d2 orbitals

What about lone pairs (on the central atom)Weve seen NH3, where they go in one of the hybrid orbitalsUsually this is the case, because the hybrid orbital represents the greatest separation of electron pairs (bonding or lone) in 3D space.They can go in other hybrid orbitals in other cases, for example..4 total pairs (2 structural, 2 lone)SeH2Valence Electron ConfigurationLewis Dot StructurePredict Electron Pair and Molecular GeometryDifferent-two lone pairs.Predict HybridizationDraw with electron pairs in the appropriate orbitals and describe the hybridizationWhat happens to the tetrahedral bond angles?Lets do this on the board.5 total pairs (4 structural, one lone)

pi () bondssigma () bonds lie along the internuclear axis between atomspi () bonds are parallel to the internuclear axis, but lie separated from it in spaceCreated from p orbitals not used in hybridizationResponsible for multiple bondsWe will concern ourselves with pi bonds in carbon containing (organic) molecules only

Double Bonds (one , one )Ethene (ethylene):C2H43 x sp2 hybrid orbitals on each carbon2 C-H sigma bonds on each carbon1 C-C sigma bond1 C-C pi bondDouble bond C-C (one sigma plus one pi)

Note that the geometryof the sp2 hybrids (trigonal planar) controls geometry about each C atom.

Triple Bonds (one , two )AcetyleneC2H22 sp hybrid orbitals on each carbon1 C-H sigma bonds on each carbon1 C-C sigma bond2 C-C pi bondsTriple bond C-C (one sigma plus two pi)

Note that the geometryof the sp hybrids (linear) controls thegeometry about each C atom.Whats different about multiple bonds (sigma + pi) vs. single (sigma) bonds?Location of the bonds Along the internuclear axis versus parallel to itSingle bonds (sigma only) can rotateMultiple bonds (sigma + pi) are rigidLeads to structural isomerscis vs. trans formsMore than one multiple (sigma + pi) in a molecule can lead to different resonance forms

Do we have cis vesus trans 1, 2 dichloroethane? Lets draw on the board.Benzene (C6H6) / Resonance6 carbons, each with 3 sp2 hybrid orbitalsEach results in 3 sigma bonds1 sigma bond to H, 2 sigma bonds to COne pi bond on each carbon atom

sp2 hybridization still controls geometry on each carbon atompi bonds make the molecule flatTwo resonance forms describe possible location of pi bondsHow does this influence bond order, bond length, bond energy?VERY stable-electrons are distributed throughout the molecule.

Molecular Orbital (MO) TheoryConsiders orbitals to be more spread out (delocalized) than Valence Bond TheorySpectroscopy results are more supportive of MO theoryIs better at describing why certain molecules do not form (why bonding does not happen in some cases)Can be used in a complimentary fashion with Valence Bond Theory to describe what is happening in molecules.We still have sigma () and pi () bonding.We add non-bonding and antibonding variablesHow to conceptualize MO theory?Instead of thinking of how hybrids form on a central atom.Think about how orbitals on two separate atoms in a bond can come together and form bonds in a moleculeCombining orbitals (MO) instead of overlapping orbitals (VBT)

What is some of this terminology? What do the symbols mean?Final Exam & RemindersOWL Deadline (Chapter 9) is the start of the final examReview Sessions (IRC 3-here)Sunday 10-11:30 AMSunday 12:30-2 PMFinal Exam (150 points)45 points Chapter 9 (short answer)128 points Chapters 2-8 (multiple choice)Pen/Pencils, Calculator, Ruler, 81/2 x 11 note sheetI will provide thermodynamic and other constantsPlancks constant, speed of light, bond enthalpies, etc.

Some important principles of MO TheoryThe number of molecular orbitals formed equals the number of atomic orbitals that have combined!Electrons are spread out over the entire moleculeAs opposed to being more localized in specific bonds as we see with VBTThe total energy of the molecular orbitals equals the sum of the energy of the atomic orbitalsIt is just that some orbitals dont contain electrons (antibonding)What about He2?

H2 forms, He2 does not. Bond Order?Bond Order = (# bonding electrons - # non-bonding electrons)

What is the bond order for H2 ?

What is the bond order for He2 ?

Bond orders less than zero suggest a molecule would not ordinarily exist. A bond order between 0 and 1 suggests that the molecule would also not exist or would be unstableOther homonuclear diatomic molecules (through F2)

Nitrogen (N2)

Bond Order ?

Diamagnetic orParamagnetic ?

Oxygen (O2)

Bond Order ?

Diamagnetic orParamagnetic ?

Heteronuclear Diatomic MoleculesThe total energy of the orbitals created is still the same as the total of the atomic orbitals, but the energy of the original incoming atomic orbitals are different.

Same rules apply in terms of # of orbitals, magnetic properties, bond order, etc.

MO vs. VB Theories..