What’s coming up??? Oct 25The atmosphere, part 1Ch. 8 Oct 27Midterm … No lecture Oct 29The atmosphere, part 2Ch. 8 Nov 1Light, blackbodies, BohrCh. 9 Nov 3,5Postulates of QM, p-in-a-boxCh. 9 Nov 8,10Hydrogen and multi – e atoms Ch. 9 Nov 12Multi-electron atomsCh.9,10 Nov 15Periodic propertiesCh. 10 Nov 17Periodic propertiesCh. 10 Nov 19Valence-bond; Lewis structures Ch. 11 Nov 22VSEPRCh. 11 Nov 24Hybrid orbitals; VSEPRCh. 11, 12 Nov 26Hybrid orbitals; MO theoryCh. 12 Nov 29MO theoryCh. 12 Dec 1bonding wrapupCh. 11,12 Dec 2Review for exam

The Final Exam December 13 (Monday) 9:00 – 12:00 Cumulative (covers everything!!) Worth 50% of total mark Multiple choice

The Final Exam From my portion, you are responsible for: –Chapter 8 … material from my lecture notes –Chapter 9 … everything –Chapter 10 … everything –Chapter 11 … everything –Chapter 12 … everything except 12.7

The Final Exam You will need to remember –Relationship between photon energy and frequency / wavelength –De Broglie AND Heisenberg relationships –Equations for energies of a particle-in-a-box AND of the hydrogen atom –VSEPR shapes AND hybribizations which give them

My office hours next week Wednesday Dec 8: AND 2-4 Friday Dec 10: AND 2-4

THE MO’s FORMED BY TWO 1s ORBITALS

The  molecular orbitals.

E Expected orbital splitting: 2s2s 2s*2s* 2s2s 2s2s 2p2p 2p*2p* 2p2p 2p2p 2p2p 2p*2p* The  do not split as much because of weaker overlap. But the s and p along the internuclear axis DO interact This pushes the  2p up..

Why does this happen? “pure” s and p orbitals O 2 F 2 Ne 2 “pure” s-p hybrids B 2 C 2 N 2  -type orbitals have different energies depending on s-p interactions

 2p orbitals  2p  2s Increasing s-p interaction

E MODIFIED ENERGY LEVEL DIAGRAM 2s2s 2s*2s* 2s2s 2s2s 2p2p 2p*2p* 2p2p 2p2p 2p2p 2p*2p*  interaction

 2p *  2p *  2p or  2p  2p  or  2p  2s *  2s Magnetism Bond order Bond E. (kJ/mol) Bond length(pm) Second row diatomic molecules B 2 Para C 2 Dia N 2 Dia O 2 Para F 2 Dia E NOTE SWITCH OF LABELS

2s2s 2s*2s* 2s2s 2s2s E 2p2p 2p*2p* 2p2p 2p2p 2p2p 2p*2p* 2s2s 2s*2s* 2s2s 2s2s 2p*2p* 2p2p 2p2p 2p2p 2p*2p* 2p2p HETERONUCLEAR DIATOMICS

Energies of 1s and 2p orbitals are very different E =  R H Z eff 2 / n 2

Electronegativities!

Outcomes of MO Model As bond order increases, bond energy increases and bond length decreases. Bond order is not absolutely associated with a particular bond energy. N 2 has a triple bond, and a correspondingly high bond energy. O 2 is paramagnetic. This is predicted by the MO model, not by the LE model, which predicts diamagnetism.

Putting our bonding models together The VSEPR / hybridization approach is good at explaining shapes around a central atom in a molecule BUT, since it depends on keeping electrons in pairs at all times, it is not so good at predicting electron distributions (like in oxygen!) Is there a way to bring them together? Let’s go back to RESONANCE

The Resonance Structures for O 3 and NO 3 -

Double bonds involve interacting p orbitals, outside of the bonding line p-  bonding spread over whole molecule p-  antibonding p-  non-bonding

We can make a similar  molecular orbital for the nitrate ion too!

Benzene - aromatic molecules

Figure 9.48 The Pi System for Benzene

Combining our 2 Models  bonds can be described as being localized.  bonding must be treated as being delocalized.