What’s coming up???• Oct 25 The atmosphere, part 1 Ch. 8• Oct 27 Midterm … No lecture• Oct 29 The atmosphere, part 2 Ch. 8• Nov 1 Light, blackbodies, Bohr Ch. 9• Nov 3,5 Postulates of QM, p-in-a-box Ch. 9• Nov 8,10Hydrogen and multi – e atoms Ch. 9• Nov 12 Multi-electron atoms Ch.9,10• Nov 15 Periodic properties Ch. 10• Nov 17 Periodic properties Ch. 10• Nov 19 Valence-bond; Lewis structures Ch. 11• Nov 22 VSEPR Ch. 11• Nov 24 Hybrid orbitals; VSEPR Ch. 11, 12• Nov 26 Hybrid orbitals; MO theory Ch. 12• Nov 29 MO theory Ch. 12• Dec 1 bonding wrapup Ch. 11,12• Dec 2 Review 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: 10-12 AND 2-4

• Friday Dec 10: 10-12 AND 2-4

THE MO’s FORMED BY TWO 1s ORBITALS

The molecular orbitals.

E

Expected orbital splitting:

2s

2s*

2s

2s

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 orbitalsO2 F2 Ne2

“pure” s-p hybridsB2 C2 N2

-type orbitals have differentenergies depending on s-p interactions

2p orbitals2p orbitals

2p

2s

Increasing s-p interaction

E

MODIFIED ENERGY LEVEL DIAGRAM

2s

2s*

2s

2s

2p

2p*

2p2p

2p

2p* interaction

2p*

2p*

2p or 2p

2por 2p

2s*

2s

Magnetism

Bond order

Bond E. (kJ/mol)

Bond length(pm)

Second row diatomic molecules

B2

Para-

1

290

159

C2

Dia-

2

620

131

N2

Dia-

3

942

110

O2

Para-

2

495

121

F2

Dia-

1

154

143

E

NOTE SWITCH OF LABELS

2s

2s*

2s

2s

E

2p

2p*

2p2p

2p

2p*

2s

2s*

2s

2s

2p*

2p

2p

2p

2p*

2p

HETERONUCLEAR DIATOMICS

Energies of 1s and 2p orbitals are very different

E = RH Zeff2 / n2

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.

• N2 has a triple bond, and a correspondingly high bond energy.

• O2 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 explainingshapes 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 O3 and NO3

-

Double bonds involve interacting p orbitals, outside of the bonding line

p- bondingspread overwhole molecule

p- antibondingp- non-bonding

We can make a similar molecular orbitalfor the nitrate ion too!

Benzene - aromatic molecules

Figure 9.48The Pi System for Benzene

Combining our 2 Models

bonds can be described as being localized.

bonding must be treated as being delocalized.