III. The Molecular Orbital ModelProblems with the L.E. ModelAssumes all e- are localized; this is not what is observedUnpaired e- are not described very well at allGives us no clue to bond energies

Molecular Orbital ModelSimilar to atomic theory, but develops orbitals for a whole moleculeGives molecular orbitals that electrons go intoProperties of the molecular orbitals (MOs)Each MO can hold 2 e- of opposite spinShape of the MO represents probability of where e- is

H2 Molecular Orbital Description1.We can construct our MOs from the AOs we already knowMO1 = 1sa + 1sb MO2 = 1sa - 1sb

Size, shape, and energy of MOs

MO1 = s-MOLarge probability between H nuclei = s interactionLower energy than H 1s orbitals = bonding MOThe 2 total valence e- pair up, giving a stabilization = bondMO2 = s*-MOLarge probability bracketing H nuclei = s interactionHigher energy than H 1s orbitals = antibonding MONo electrons in this MOs1ss*1s

5.H2- has three electronsLess stable than H2 One e- in antibonding MO

Bond Order = Difference between the number of bonding electrons and number of antibonding electrons divided by two.

Example of B. O.H2 = (2 0)/2 = 1 (single bond strength)H2- = (2 1)/2 = 0.5 (less than single bond strength, but stable)He2 = (2 2)/2 = 0 (not stable, not a diatomic molecule)

I. Bonding in Homonuclear DiatomicsLi, Be, BLi2 MO descriptionLi 1s22s1, but we can only use valence e- (2s) in bonding1s orbital is core and cant be reached for bondingMOs for Li2

Bond Order = (2 0)/2 = 1, so it is stable

MO electron configuration would be s2s2 Li2 is stable, but only occurs as a gas

Be2 uses the same MOs as Li2but is s2s2 s*2s2 Bond Order = (2-2)/2 = 0Be2 is not a stable diatomicB2 must begin to use MOs generated from 2p AOsB = 1s22s22p1 p-orbital MO combinations

c.Head-on overlap gives s2p and s*2p interactions

Side-on overlap gives p2p and p*2p interactions.There are 2 such interactions each, because of the 2Perpendicular p-orbitals not involved in the s-bond.

2s--2s overlap is more favorable, at lower energy

Bond Order = (4 2)/2 = 1, stable diatomic

IV.MagnetismA.Magnetic Balance

Diamagnetic = repelled by a magnetic fieldPaired e- are diamagneticH2 is diamagnetic because all of its e- are pairedMost compounds are diamagnetic because paired e- are favorableRepulsion is very weak

Paramagnetic = attracted by a magnetic fielda.unpaired electrons are paramagneticb.much stronger than diamagnetismc.Relatively few compounds have unpaired e-

B.Magnetism and MO Theory1.We predicted from MO theory that B2 would be diamagnetic2.Experiment shows us that it is paramagnetic.3.We must adjust our model to explain this

We must allow our model to take into account 2s/2p interaction2s/2p mixing changes the ordering of some of our MOsSpacing is no longer symmetricPredicts paramagnetism of B2 d. Bond Order is still 1

Other homonuclear diatomicsC2 and N2 use same ordering and MO set as B2O2 and F2 have different ordering because s/p mixing changes

Trends in 2nd row diatomicsAs Bond Order increase, Bond Energy increases, Bond Length decreasesBond Order doesnt completely predict Bond EnergyB2 > F2 even though both have BO = 1F2 has much electron repulsion, weakening the bondN2 bond order = (8-2)/2 = 3. This triple bond is very strong and explains why N2 is very unreactive.O2 is paramagnetic.

V.Heteronuclear Diatomics: Different elements have different A.O. energiesA.Adjacent elements are the most similar (NO, CN, etc)We can use same MOs as for homonuclear diatomics for adjacentsNO Bond Order = (8-3)/2 = 2.5

MO Theory better handles unpaired e-Paramagnetic

5.NO+ and CN- have the same configurationBO = (8-2)/2 = 3 (triple bonds)We can use the same figureDiamagnetic

Elements far apart have very different energiesWe must devise new MO ordering for each caseHF (use H1s and F2p)F much lower energy (more tightly bound e-)F2p < H1s so the electrons will be closer to F (Predicts Electronegativity!!!)

VI. Combining LE and MO ModelsResonance StructuresL.E. model is not very accurate in some cases where more than one Lewis Structure can be drawnResonance structures = equivalent, but not exact representations of the bonding in a molecule

O3

NO3-

Benzene

s-bonds can be localized without problems; Use LE Model p-bonds need to Delocalize in some molecules for stabilityStill use LE Model to describe the s-bondingUse MO model to describe p-bondingSimple as possible, but must be accurateBenzene

4. NO3-