Tuesday, November 27, 2007

2007 Chapter 12

Questions on the material covered in Chapter 12? Leave a comment here.

12 comments:

chris ward said...

Hey you mentioned in class the student union is selling old chem exams? where was that again?

Scott McIndoe said...

Er, it WAS today... maybe tomorrow as well? If you miss out on the ones with model answers, don't worry - the unmarked ones from ZAP are also useful. If you can't figure out a particular problem, ask me (or the TAs at the drop-in).

Anonymous said...

I am not in your class but we wasn't informed about any exams for sale. Any chance I could get them? The one's from ZAP are pretty old and some sections are out of our syllabus.

Scott McIndoe said...

You'll have to check with the Chemistry Student Society (chemsoc@uvic.ca) - I'm not sure if they're selling them any other days.

Anonymous said...

Can a valence band be considered as a large group of bonding MO's of which there are so many the energy "distance" between them is close to zero? Also, then, could a conduction band be considered a large group of antibonding MO's of which there are so many the energy "distance" between each is neglegible?

If this is the case, how does the "band gap" arise in different magnitudes in different substances? Is it simply because of the atomic bonding? I.e. insulators have ionic bonds which allow very little sharing of electrons (practically none), so the gap between bonding and antibonding orbitals is sharply defined (insulators such as metal oxides)?

So for a metal the conduction band is more or less drawn in just to remind us that it is there. That is, electrons will more or less never enter it because there is so much room in the valence band? Or is there truly no gap between antibonding and bonding MO's because the electrons are so delocalized? I.e. an electron might conduct across the conduction band just as happily as across the valence band?

That leads me to another question: why is the conduction band named so? Is it because for conduction to happen, electrons must moved across it? Would this not defeat the argument of doping semi-conductors with elements with less than 4 (usually) valence electrons to create holes in the valence band?

Hope that made sense. Thanks for the help.

Scott McIndoe said...

Each paragraph in turn...

Yes, that is an OK description.

The band gap arises because of the inherent difference in energies of the bonding and antibonding orbitals to begin with. Diamond, for example, forms very strong sp3 bonds with 4 surrounding carbon atoms, and the bonding and antibonding orbitals are very widely separated - so even generating a broad band doesn't diminish the band gap enough to stop it being an insulator. Silicon, on the other hand, forms longer, weaker bonds and so the separation between bonding and antibonding is less and hence Si is a semiconductor.

In a metal, we have a partially-filled band, not a different band entirely, so there is no band gap as such and it makes more sense to think about it as a partly filled valence band. You'll notice that in diagrams it is not usually labelled CB, VB etc. for metals.

You're right - the CB, VB notation doesn't really cope well with the idea of doping.

Anonymous said...

Which were the sections of chapter 12 that will not be on the exam?

Btw, I think you're a great prof-very clear & easy to understand. Why aren't you teaching 102? :(

Scott McIndoe said...

12.3 Biomaterials and 12.6 Nanomaterials.
Thanks... my other job is to get some research done! I'd encourage you to stick with chemistry and I'll see you again in 4th year for some advanced inorganic chemistry :)

Anonymous said...

Do we need to memorize the names and structures of all the polymers mentioned in our notes--such as polyethylene, polypropylene, polyurethane, etc?

Scott McIndoe said...

Well, some of them are easy - PE, PP for example! but no, we don't expect you to memorize them all. We do, however, expect that if you're given a monomer(s) you can draw the polymer and vice versa.

Cassandra said...

I am just curious... i dont see anything in the notes about "thin films", however, there are suggested problems for both this subtopic, as well as for bio-materials... just so i am clear, are either of these on the final?

Scott McIndoe said...

Neither thin films (not taught this year) nor biomaterials (taught to our section but not others) will be examined this year.